Remove forward() method from common.policies.BaseModel (#1061)

* Remove forward() method.

* Updated changelog

docs/misc/changelog.rst

@@ -29,6 +29,7 @@ Bug Fixes:
 - Fixed missing verbose parameter passing in the ``EvalCallback`` constructor (@burakdmb)
 - Fixed the issue that when updating the target network in DQN, SAC, TD3, the ``running_mean`` and ``running_var`` properties of batch norm layers are not updated (@honglu2875)
 - Fixed incorrect type annotation of the replay_buffer_class argument in ``common.OffPolicyAlgorithm`` initializer, where an instance instead of a class was required (@Rocamonde)
+- Removed ``forward()`` abstract method declaration from ``common.policies.BaseModel`` (already defined in ``torch.nn.Module``) to fix type errors in subclasses (@Rocamonde)
 
 Deprecations:
 ^^^^^^^^^^^^^

stable_baselines3/common/policies.py

@@ -87,10 +87,6 @@ class BaseModel(nn.Module, ABC):
         self.features_extractor_class = features_extractor_class
         self.features_extractor_kwargs = features_extractor_kwargs
 
-    @abstractmethod
-    def forward(self, *args, **kwargs):
-        pass
-
     def _update_features_extractor(
         self,
         net_kwargs: Dict[str, Any],