diff --git a/setup.py b/setup.py
index 8104892..863eb20 100644
--- a/setup.py
+++ b/setup.py
@@ -67,7 +67,7 @@ setup(
             "pydocstyle",
             "doc8",
         ],
-        "atari": ["atari_py", "cv2"],
+        "atari": ["atari_py", "opencv-python"],
         "mujoco": ["mujoco_py"],
         "pybullet": ["pybullet"],
     },
diff --git a/tianshou/trainer/offline.py b/tianshou/trainer/offline.py
index aad7299..a2bcf05 100644
--- a/tianshou/trainer/offline.py
+++ b/tianshou/trainer/offline.py
@@ -1,6 +1,5 @@
 import time
 import tqdm
-import warnings
 import numpy as np
 from collections import defaultdict
 from typing import Dict, Union, Callable, Optional
@@ -68,9 +67,6 @@ def offline_trainer(
 
     :return: See :func:`~tianshou.trainer.gather_info`.
     """
-    if save_fn:
-        warnings.warn("Please consider using save_checkpoint_fn instead of save_fn.")
-
     start_epoch, gradient_step = 0, 0
     if resume_from_log:
         start_epoch, _, gradient_step = logger.restore_data()
diff --git a/tianshou/trainer/offpolicy.py b/tianshou/trainer/offpolicy.py
index 5df2d6f..2a576cc 100644
--- a/tianshou/trainer/offpolicy.py
+++ b/tianshou/trainer/offpolicy.py
@@ -1,6 +1,5 @@
 import time
 import tqdm
-import warnings
 import numpy as np
 from collections import defaultdict
 from typing import Dict, Union, Callable, Optional
@@ -83,9 +82,6 @@ def offpolicy_trainer(
 
     :return: See :func:`~tianshou.trainer.gather_info`.
     """
-    if save_fn:
-        warnings.warn("Please consider using save_checkpoint_fn instead of save_fn.")
-
     start_epoch, env_step, gradient_step = 0, 0, 0
     if resume_from_log:
         start_epoch, env_step, gradient_step = logger.restore_data()
diff --git a/tianshou/trainer/onpolicy.py b/tianshou/trainer/onpolicy.py
index 379aba6..1696421 100644
--- a/tianshou/trainer/onpolicy.py
+++ b/tianshou/trainer/onpolicy.py
@@ -1,6 +1,5 @@
 import time
 import tqdm
-import warnings
 import numpy as np
 from collections import defaultdict
 from typing import Dict, Union, Callable, Optional
@@ -89,9 +88,6 @@ def onpolicy_trainer(
 
         Only either one of step_per_collect and episode_per_collect can be specified.
     """
-    if save_fn:
-        warnings.warn("Please consider using save_checkpoint_fn instead of save_fn.")
-
     start_epoch, env_step, gradient_step = 0, 0, 0
     if resume_from_log:
         start_epoch, env_step, gradient_step = logger.restore_data()