Fix to_numpy. (#73)

Co-authored-by: Alexis Duburcq <alexis.duburcq@wandercraft.eu>

tianshou/data/batch.py

@@ -95,7 +95,7 @@ class Batch:
                     k__ = '_' + k + '@' + k_
                     self.__dict__[k__] = v_
             else:
-                self.__dict__[k] = kwargs[k]
+                self.__dict__[k] = v
 
     def __getstate__(self):
         """Pickling interface. Only the actual data are serialized
@@ -180,7 +180,7 @@ class Batch:
         """
         for k, v in self.__dict__.items():
             if isinstance(v, torch.Tensor):
-                self.__dict__[k] = v.cpu().numpy()
+                self.__dict__[k] = v.detach().cpu().numpy()
             elif isinstance(v, Batch):
                 v.to_numpy()