diff --git a/README.md b/README.md
index 2c027d5..2c5db38 100644
--- a/README.md
+++ b/README.md
@@ -139,12 +139,14 @@ Check out the [GitHub Actions](https://github.com/thu-ml/tianshou/actions) page
 
 ### Modularized Policy
 
-We decouple all of the algorithms into 4 parts:
+We decouple all of the algorithms roughly into the following parts:
 
 - `__init__`: initialize the policy;
 - `forward`: to compute actions over given observations;
 - `process_fn`: to preprocess data from replay buffer (since we have reformulated all algorithms to replay-buffer based algorithms);
-- `learn`: to learn from a given batch data.
+- `learn`: to learn from a given batch data;
+- `post_process_fn`: to update the replay buffer from the learning process (e.g., prioritized replay buffer needs to update the weight);
+- `update`: the main interface for training, i.e., `process_fn -> learn -> post_process_fn`.
 
 Within this API, we can interact with different policies conveniently.
 
@@ -165,7 +167,7 @@ result = collector.collect(n_episode=[1, 0, 3])
 If you want to train the given policy with a sampled batch:
 
 ```python
-result = policy.learn(collector.sample(batch_size))
+result = policy.update(batch_size, collector.buffer)
 ```
 
 You can check out the [documentation](https://tianshou.readthedocs.io) for further usage.
diff --git a/docs/tutorials/concepts.rst b/docs/tutorials/concepts.rst
index 3f033ea..ba771ad 100644
--- a/docs/tutorials/concepts.rst
+++ b/docs/tutorials/concepts.rst
@@ -64,12 +64,14 @@ Policy
 
 Tianshou aims to modularizing RL algorithms. It comes into several classes of policies in Tianshou. All of the policy classes must inherit :class:`~tianshou.policy.BasePolicy`.
 
-A policy class typically has four parts:
+A policy class typically has the following parts:
 
-* :meth:`~tianshou.policy.BasePolicy.__init__`: initialize the policy, including coping the target network and so on;
+* :meth:`~tianshou.policy.BasePolicy.__init__`: initialize the policy, including copying the target network and so on;
 * :meth:`~tianshou.policy.BasePolicy.forward`: compute action with given observation;
-* :meth:`~tianshou.policy.BasePolicy.process_fn`: pre-process data from the replay buffer (this function can interact with replay buffer);
+* :meth:`~tianshou.policy.BasePolicy.process_fn`: pre-process data from the replay buffer;
 * :meth:`~tianshou.policy.BasePolicy.learn`: update policy with a given batch of data.
+* :meth:`~tianshou.policy.BasePolicy.post_process_fn`: update the buffer with a given batch of data.
+* :meth:`~tianshou.policy.BasePolicy.update`: the main interface for training. This function samples data from buffer, pre-process data (such as computing n-step return), learn with the data, and finally post-process the data (such as updating prioritized replay buffer); in short, ``process_fn -> learn -> post_process_fn``.
 
 Take 2-step return DQN as an example. The 2-step return DQN compute each frame's return as:
 
@@ -125,10 +127,8 @@ Collector
 ---------
 
 The :class:`~tianshou.data.Collector` enables the policy to interact with different types of environments conveniently.
-In short, :class:`~tianshou.data.Collector` has two main methods:
 
-* :meth:`~tianshou.data.Collector.collect`: let the policy perform (at least) a specified number of step ``n_step`` or episode ``n_episode`` and store the data in the replay buffer;
-* :meth:`~tianshou.data.Collector.sample`: sample a data batch from replay buffer; it will call :meth:`~tianshou.policy.BasePolicy.process_fn` before returning the final batch data.
+:class:`~tianshou.data.Collector` has one main method :meth:`~tianshou.data.Collector.collect`: it let the policy perform (at least) a specified number of step ``n_step`` or episode ``n_episode`` and store the data in the replay buffer.
 
 Why do we mention **at least** here? For multiple environments, we could not directly store the collected data into the replay buffer, since it breaks the principle of storing data chronologically.
 
@@ -144,8 +144,6 @@ Once you have a collector and a policy, you can start writing the training metho
 
 Tianshou has two types of trainer: :func:`~tianshou.trainer.onpolicy_trainer` and :func:`~tianshou.trainer.offpolicy_trainer`, corresponding to on-policy algorithms (such as Policy Gradient) and off-policy algorithms (such as DQN). Please check out :doc:`/api/tianshou.trainer` for the usage.
 
-There will be more types of trainers, for instance, multi-agent trainer.
-
 
 .. _pseudocode:
 
@@ -165,7 +163,8 @@ We give a high-level explanation through the pseudocode used in section :ref:`po
         buffer.store(s, a, s_, r, d)                                # collector.collect(...)
         s = s_                                                      # collector.collect(...)
         if i % 1000 == 0:                                           # done in trainer
-            b_s, b_a, b_s_, b_r, b_d = buffer.get(size=64)          # collector.sample(batch_size)
+                                                                    # the following is done in policy.update(batch_size, buffer)
+            b_s, b_a, b_s_, b_r, b_d = buffer.get(size=64)          # buffer.sample(batch_size)
             # compute 2-step returns. How?
             b_ret = compute_2_step_return(buffer, b_r, b_d, ...)    # policy.process_fn(batch, buffer, indice)
             # update DQN policy
diff --git a/docs/tutorials/dqn.rst b/docs/tutorials/dqn.rst
index e5edbfd..764bb45 100644
--- a/docs/tutorials/dqn.rst
+++ b/docs/tutorials/dqn.rst
@@ -210,8 +210,8 @@ Tianshou supports user-defined training code. Here is the code snippet:
                 # back to training eps
                 policy.set_eps(0.1)
 
-        # train policy with a sampled batch data
-        losses = policy.learn(train_collector.sample(batch_size=64))
+        # train policy with a sampled batch data from buffer
+        losses = policy.update(64, train_collector.buffer)
 
 For further usage, you can refer to the :doc:`/tutorials/cheatsheet`.
 
diff --git a/test/base/test_collector.py b/test/base/test_collector.py
index 68cf5c7..38e5d93 100644
--- a/test/base/test_collector.py
+++ b/test/base/test_collector.py
@@ -174,7 +174,7 @@ def test_collector_with_dict_state():
     c1.seed(0)
     c1.collect(n_step=10)
     c1.collect(n_episode=[2, 1, 1, 2])
-    batch = c1.sample(10)
+    batch, _ = c1.buffer.sample(10)
     print(batch)
     c0.buffer.update(c1.buffer)
     assert np.allclose(c0.buffer[:len(c0.buffer)].obs.index, np.expand_dims([
@@ -184,7 +184,7 @@ def test_collector_with_dict_state():
     c2 = Collector(policy, envs, ReplayBuffer(size=100, stack_num=4),
                    Logger.single_preprocess_fn)
     c2.collect(n_episode=[0, 0, 0, 10])
-    batch = c2.sample(10)
+    batch, _ = c2.buffer.sample(10)
     print(batch['obs_next']['index'])
 
 
@@ -209,7 +209,7 @@ def test_collector_with_ma():
     assert np.asanyarray(r).size == 1 and r == 4.
     r = c1.collect(n_episode=[2, 1, 1, 2])['rew']
     assert np.asanyarray(r).size == 1 and r == 4.
-    batch = c1.sample(10)
+    batch, _ = c1.buffer.sample(10)
     print(batch)
     c0.buffer.update(c1.buffer)
     obs = np.array(np.expand_dims([
@@ -226,7 +226,7 @@ def test_collector_with_ma():
                    Logger.single_preprocess_fn, reward_metric=reward_metric)
     r = c2.collect(n_episode=[0, 0, 0, 10])['rew']
     assert np.asanyarray(r).size == 1 and r == 4.
-    batch = c2.sample(10)
+    batch, _ = c2.buffer.sample(10)
     print(batch['obs_next'])
 
 
diff --git a/tianshou/data/collector.py b/tianshou/data/collector.py
index 9105374..8d2863c 100644
--- a/tianshou/data/collector.py
+++ b/tianshou/data/collector.py
@@ -64,16 +64,6 @@ class Collector(object):
         #   sleep time between rendering consecutive frames)
         collector.collect(n_episode=1, render=0.03)
 
-        # sample data with a given number of batch-size:
-        batch_data = collector.sample(batch_size=64)
-        # policy.learn(batch_data)  # btw, vanilla policy gradient only
-        #   supports on-policy training, so here we pick all data in the buffer
-        batch_data = collector.sample(batch_size=0)
-        policy.learn(batch_data)
-        # on-policy algorithms use the collected data only once, so here we
-        #   clear the buffer
-        collector.reset_buffer()
-
     Collected data always consist of full episodes. So if only ``n_step``
     argument is give, the collector may return the data more than the
     ``n_step`` limitation. Same as ``n_episode`` for the multiple environment
@@ -357,13 +347,18 @@ class Collector(object):
 
     def sample(self, batch_size: int) -> Batch:
         """Sample a data batch from the internal replay buffer. It will call
-        :meth:`~tianshou.policy.BasePolicy.process_fn` before returning
-        the final batch data.
+        :meth:`~tianshou.policy.BasePolicy.process_fn` before returning the
+        final batch data.
 
         :param int batch_size: ``0`` means it will extract all the data from
             the buffer, otherwise it will extract the data with the given
             batch_size.
         """
+        import warnings
+        warnings.warn(
+            'Collector.sample is deprecated and will cause error if you use '
+            'prioritized experience replay! Collector.sample will be removed '
+            'upon version 0.3. Use policy.update instead!', Warning)
         batch_data, indice = self.buffer.sample(batch_size)
         batch_data = self.process_fn(batch_data, self.buffer, indice)
         return batch_data
diff --git a/tianshou/policy/base.py b/tianshou/policy/base.py
index cc1a593..a2f545e 100644
--- a/tianshou/policy/base.py
+++ b/tianshou/policy/base.py
@@ -93,9 +93,8 @@ class BasePolicy(ABC, nn.Module):
 
             # some code
             return Batch(..., policy=Batch(log_prob=dist.log_prob(act)))
-            # and in the sampled data batch, you can directly call
-            # batch.policy.log_prob to get your data, although it is stored in
-            # np.ndarray.
+            # and in the sampled data batch, you can directly use
+            # batch.policy.log_prob to get your data.
         """
         pass
 
@@ -123,6 +122,7 @@ class BasePolicy(ABC, nn.Module):
         v_s_: Optional[Union[np.ndarray, torch.Tensor]] = None,
         gamma: float = 0.99,
         gae_lambda: float = 0.95,
+        rew_norm: bool = False,
     ) -> Batch:
         """Compute returns over given full-length episodes, including the
         implementation of Generalized Advantage Estimator (arXiv:1506.02438).
@@ -136,6 +136,8 @@ class BasePolicy(ABC, nn.Module):
             to 0.99.
         :param float gae_lambda: the parameter for Generalized Advantage
             Estimation, should be in [0, 1], defaults to 0.95.
+        :param bool rew_norm: normalize the reward to Normal(0, 1), defaults
+            to ``False``.
 
         :return: a Batch. The result will be stored in batch.returns as a numpy
             array with shape (bsz, ).
@@ -150,6 +152,8 @@ class BasePolicy(ABC, nn.Module):
         for i in range(len(rew) - 1, -1, -1):
             gae = delta[i] + m[i] * gae
             returns[i] += gae
+        if rew_norm and not np.isclose(returns.std(), 0, 1e-2):
+            returns = (returns - returns.mean()) / returns.std()
         batch.returns = returns
         return batch
 
@@ -196,7 +200,7 @@ class BasePolicy(ABC, nn.Module):
         if rew_norm:
             bfr = rew[:min(len(buffer), 1000)]  # avoid large buffer
             mean, std = bfr.mean(), bfr.std()
-            if np.isclose(std, 0):
+            if np.isclose(std, 0, 1e-2):
                 mean, std = 0, 1
         else:
             mean, std = 0, 1
@@ -216,9 +220,30 @@ class BasePolicy(ABC, nn.Module):
         batch.returns = target_q * gammas + returns
         # prio buffer update
         if isinstance(buffer, PrioritizedReplayBuffer):
-            batch.update_weight = buffer.update_weight
-            batch.indice = indice
             batch.weight = to_torch_as(batch.weight, target_q)
         else:
             batch.weight = torch.ones_like(target_q)
         return batch
+
+    def post_process_fn(self, batch: Batch,
+                        buffer: ReplayBuffer, indice: np.ndarray):
+        """Post-process the data from the provided replay buffer. Typical
+        usage is to update the sampling weight in prioritized experience
+        replay. Check out :ref:`policy_concept` for more information.
+        """
+        if isinstance(buffer, PrioritizedReplayBuffer):
+            buffer.update_weight(indice, batch.weight)
+
+    def update(self, batch_size: int, buffer: ReplayBuffer, *args, **kwargs):
+        """Update the policy network and replay buffer (if needed). It includes
+        three function steps: process_fn, learn, and post_process_fn.
+
+        :param int batch_size: 0 means it will extract all the data from the
+            buffer, otherwise it will sample a batch with the given batch_size.
+        :param ReplayBuffer buffer: the corresponding replay buffer.
+        """
+        batch, indice = buffer.sample(batch_size)
+        batch = self.process_fn(batch, buffer, indice)
+        result = self.learn(batch, *args, **kwargs)
+        self.post_process_fn(batch, buffer, indice)
+        return result
diff --git a/tianshou/policy/modelfree/a2c.py b/tianshou/policy/modelfree/a2c.py
index 2a5c123..52d8dd2 100644
--- a/tianshou/policy/modelfree/a2c.py
+++ b/tianshou/policy/modelfree/a2c.py
@@ -67,7 +67,8 @@ class A2CPolicy(PGPolicy):
                 v_.append(to_numpy(self.critic(b.obs_next)))
         v_ = np.concatenate(v_, axis=0)
         return self.compute_episodic_return(
-            batch, v_, gamma=self._gamma, gae_lambda=self._lambda)
+            batch, v_, gamma=self._gamma, gae_lambda=self._lambda,
+            rew_norm=self._rew_norm)
 
     def forward(self, batch: Batch,
                 state: Optional[Union[dict, Batch, np.ndarray]] = None,
@@ -97,9 +98,6 @@ class A2CPolicy(PGPolicy):
     def learn(self, batch: Batch, batch_size: int, repeat: int,
               **kwargs) -> Dict[str, List[float]]:
         self._batch = batch_size
-        r = batch.returns
-        if self._rew_norm and not np.isclose(r.std(), 0):
-            batch.returns = (r - r.mean()) / r.std()
         losses, actor_losses, vf_losses, ent_losses = [], [], [], []
         for _ in range(repeat):
             for b in batch.split(batch_size):
diff --git a/tianshou/policy/modelfree/ddpg.py b/tianshou/policy/modelfree/ddpg.py
index 2205102..79a65d3 100644
--- a/tianshou/policy/modelfree/ddpg.py
+++ b/tianshou/policy/modelfree/ddpg.py
@@ -144,10 +144,8 @@ class DDPGPolicy(BasePolicy):
         current_q = self.critic(batch.obs, batch.act).flatten()
         target_q = batch.returns.flatten()
         td = current_q - target_q
-        if hasattr(batch, 'update_weight'):  # prio-buffer
-            batch.update_weight(batch.indice, td)
         critic_loss = (td.pow(2) * batch.weight).mean()
-        # critic_loss = F.mse_loss(current_q, target_q)
+        batch.weight = td  # prio-buffer
         self.critic_optim.zero_grad()
         critic_loss.backward()
         self.critic_optim.step()
diff --git a/tianshou/policy/modelfree/dqn.py b/tianshou/policy/modelfree/dqn.py
index c37dac5..f1a01a6 100644
--- a/tianshou/policy/modelfree/dqn.py
+++ b/tianshou/policy/modelfree/dqn.py
@@ -160,10 +160,8 @@ class DQNPolicy(BasePolicy):
         q = q[np.arange(len(q)), batch.act]
         r = to_torch_as(batch.returns, q).flatten()
         td = r - q
-        if hasattr(batch, 'update_weight'):  # prio-buffer
-            batch.update_weight(batch.indice, td)
         loss = (td.pow(2) * batch.weight).mean()
-        # loss = F.mse_loss(q, r)
+        batch.weight = td  # prio-buffer
         loss.backward()
         self.optim.step()
         self._cnt += 1
diff --git a/tianshou/policy/modelfree/pg.py b/tianshou/policy/modelfree/pg.py
index d6176e6..8fded95 100644
--- a/tianshou/policy/modelfree/pg.py
+++ b/tianshou/policy/modelfree/pg.py
@@ -51,7 +51,7 @@ class PGPolicy(BasePolicy):
         # batch.returns = self._vectorized_returns(batch)
         # return batch
         return self.compute_episodic_return(
-            batch, gamma=self._gamma, gae_lambda=1.)
+            batch, gamma=self._gamma, gae_lambda=1., rew_norm=self._rew_norm)
 
     def forward(self, batch: Batch,
                 state: Optional[Union[dict, Batch, np.ndarray]] = None,
@@ -81,9 +81,6 @@ class PGPolicy(BasePolicy):
     def learn(self, batch: Batch, batch_size: int, repeat: int,
               **kwargs) -> Dict[str, List[float]]:
         losses = []
-        r = batch.returns
-        if self._rew_norm and not np.isclose(r.std(), 0):
-            batch.returns = (r - r.mean()) / r.std()
         for _ in range(repeat):
             for b in batch.split(batch_size):
                 self.optim.zero_grad()
diff --git a/tianshou/policy/modelfree/ppo.py b/tianshou/policy/modelfree/ppo.py
index 2d1dece..3094be8 100644
--- a/tianshou/policy/modelfree/ppo.py
+++ b/tianshou/policy/modelfree/ppo.py
@@ -79,18 +79,29 @@ class PPOPolicy(PGPolicy):
                    indice: np.ndarray) -> Batch:
         if self._rew_norm:
             mean, std = batch.rew.mean(), batch.rew.std()
-            if not np.isclose(std, 0):
+            if not np.isclose(std, 0, 1e-2):
                 batch.rew = (batch.rew - mean) / std
-        if self._lambda in [0, 1]:
-            return self.compute_episodic_return(
-                batch, None, gamma=self._gamma, gae_lambda=self._lambda)
-        v_ = []
+        v, v_, old_log_prob = [], [], []
         with torch.no_grad():
             for b in batch.split(self._batch, shuffle=False):
                 v_.append(self.critic(b.obs_next))
+                v.append(self.critic(b.obs))
+                old_log_prob.append(self(b).dist.log_prob(
+                    to_torch_as(b.act, v[0])))
         v_ = to_numpy(torch.cat(v_, dim=0))
-        return self.compute_episodic_return(
-            batch, v_, gamma=self._gamma, gae_lambda=self._lambda)
+        batch = self.compute_episodic_return(
+            batch, v_, gamma=self._gamma, gae_lambda=self._lambda,
+            rew_norm=self._rew_norm)
+        batch.v = torch.cat(v, dim=0).flatten()  # old value
+        batch.act = to_torch_as(batch.act, v[0])
+        batch.logp_old = torch.cat(old_log_prob, dim=0)
+        batch.returns = to_torch_as(batch.returns, v[0])
+        batch.adv = batch.returns - batch.v
+        if self._rew_norm:
+            mean, std = batch.adv.mean(), batch.adv.std()
+            if not np.isclose(std.item(), 0, 1e-2):
+                batch.adv = (batch.adv - mean) / std
+        return batch
 
     def forward(self, batch: Batch,
                 state: Optional[Union[dict, Batch, np.ndarray]] = None,
@@ -123,26 +134,6 @@ class PPOPolicy(PGPolicy):
               **kwargs) -> Dict[str, List[float]]:
         self._batch = batch_size
         losses, clip_losses, vf_losses, ent_losses = [], [], [], []
-        v = []
-        old_log_prob = []
-        with torch.no_grad():
-            for b in batch.split(batch_size, shuffle=False):
-                v.append(self.critic(b.obs))
-                old_log_prob.append(self(b).dist.log_prob(
-                    to_torch_as(b.act, v[0])))
-        batch.v = torch.cat(v, dim=0).flatten()  # old value
-        batch.act = to_torch_as(batch.act, v[0])
-        batch.logp_old = torch.cat(old_log_prob, dim=0)
-        batch.returns = to_torch_as(batch.returns, v[0])
-        if self._rew_norm:
-            mean, std = batch.returns.mean(), batch.returns.std()
-            if not np.isclose(std.item(), 0):
-                batch.returns = (batch.returns - mean) / std
-        batch.adv = batch.returns - batch.v
-        if self._rew_norm:
-            mean, std = batch.adv.mean(), batch.adv.std()
-            if not np.isclose(std.item(), 0):
-                batch.adv = (batch.adv - mean) / std
         for _ in range(repeat):
             for b in batch.split(batch_size):
                 dist = self(b).dist
diff --git a/tianshou/policy/modelfree/sac.py b/tianshou/policy/modelfree/sac.py
index ce4a5ba..341fe7b 100644
--- a/tianshou/policy/modelfree/sac.py
+++ b/tianshou/policy/modelfree/sac.py
@@ -154,9 +154,7 @@ class SACPolicy(DDPGPolicy):
         self.critic2_optim.zero_grad()
         critic2_loss.backward()
         self.critic2_optim.step()
-        # prio-buffer
-        if hasattr(batch, 'update_weight'):
-            batch.update_weight(batch.indice, (td1 + td2) / 2.)
+        batch.weight = (td1 + td2) / 2.  # prio-buffer
         # actor
         obs_result = self(batch, explorating=False)
         a = obs_result.act
diff --git a/tianshou/policy/modelfree/td3.py b/tianshou/policy/modelfree/td3.py
index 698145f..9a34095 100644
--- a/tianshou/policy/modelfree/td3.py
+++ b/tianshou/policy/modelfree/td3.py
@@ -132,8 +132,7 @@ class TD3Policy(DDPGPolicy):
         self.critic2_optim.zero_grad()
         critic2_loss.backward()
         self.critic2_optim.step()
-        if hasattr(batch, 'update_weight'):  # prio-buffer
-            batch.update_weight(batch.indice, (td1 + td2) / 2.)
+        batch.weight = (td1 + td2) / 2.  # prio-buffer
         if self._cnt % self._freq == 0:
             actor_loss = -self.critic1(
                 batch.obs, self(batch, eps=0).act).mean()
diff --git a/tianshou/trainer/offpolicy.py b/tianshou/trainer/offpolicy.py
index 408d4e7..171cbb9 100644
--- a/tianshou/trainer/offpolicy.py
+++ b/tianshou/trainer/offpolicy.py
@@ -28,7 +28,8 @@ def offpolicy_trainer(
         verbose: bool = True,
         test_in_train: bool = True,
 ) -> Dict[str, Union[float, str]]:
-    """A wrapper for off-policy trainer procedure.
+    """A wrapper for off-policy trainer procedure. The ``step`` in trainer
+    means a policy network update.
 
     :param policy: an instance of the :class:`~tianshou.policy.BasePolicy`
         class.
@@ -47,8 +48,9 @@ def offpolicy_trainer(
     :param int batch_size: the batch size of sample data, which is going to
         feed in the policy network.
     :param int update_per_step: the number of times the policy network would
-        be updated after frames be collected. In other words, collect some
-        frames and do some policy network update.
+        be updated after frames are collected, for example, set it to 256 means
+        it updates policy 256 times once after ``collect_per_step`` frames are
+        collected.
     :param function train_fn: a function receives the current number of epoch
         index and performs some operations at the beginning of training in this
         epoch.
@@ -103,7 +105,7 @@ def offpolicy_trainer(
                 for i in range(update_per_step * min(
                         result['n/st'] // collect_per_step, t.total - t.n)):
                     global_step += 1
-                    losses = policy.learn(train_collector.sample(batch_size))
+                    losses = policy.update(batch_size, train_collector.buffer)
                     for k in result.keys():
                         data[k] = f'{result[k]:.2f}'
                         if writer and global_step % log_interval == 0:
diff --git a/tianshou/trainer/onpolicy.py b/tianshou/trainer/onpolicy.py
index dec42b2..e31724d 100644
--- a/tianshou/trainer/onpolicy.py
+++ b/tianshou/trainer/onpolicy.py
@@ -28,7 +28,8 @@ def onpolicy_trainer(
         verbose: bool = True,
         test_in_train: bool = True,
 ) -> Dict[str, Union[float, str]]:
-    """A wrapper for on-policy trainer procedure.
+    """A wrapper for on-policy trainer procedure. The ``step`` in trainer means
+    a policy network update.
 
     :param policy: an instance of the :class:`~tianshou.policy.BasePolicy`
         class.
@@ -101,8 +102,8 @@ def onpolicy_trainer(
                         policy.train()
                         if train_fn:
                             train_fn(epoch)
-                losses = policy.learn(
-                    train_collector.sample(0), batch_size, repeat_per_collect)
+                losses = policy.update(
+                    0, train_collector.buffer, batch_size, repeat_per_collect)
                 train_collector.reset_buffer()
                 step = 1
                 for k in losses.keys():