Model state should support PyTorch API (#727)

Summary:
Pull Request resolved: #727

Classy Models should work like regular PyTorch models. The `{get, set}_classy_state` functions for state are the only blockers which this diff fixes by moving over to `state_dict` and `load_state_dict`.

`{get, set}_classy_state` will still work for backwards compatibility, but will call the PyTorch functions directly.

Differential Revision: D25213283

fbshipit-source-id: 3cd64f530de83574174884d3b8848a1f6003f854

classy_vision/models/classy_model.py

@@ -201,7 +201,7 @@ def from_checkpoint(cls, checkpoint):
         model.set_classy_state(checkpoint["classy_state_dict"]["base_model"])
         return model
 
-    def get_classy_state(self, deep_copy=False):
+    def state_dict(self, deep_copy=False):
         """Get the state of the ClassyModel.
 
         The returned state is used for checkpointing.
@@ -222,7 +222,7 @@ def get_classy_state(self, deep_copy=False):
         # as the trunk state. If the model doesn't have heads attached, all of the
         # model's state lives in the trunk.
         self.clear_heads()
-        trunk_state_dict = self.state_dict()
+        trunk_state_dict = super().state_dict()
         self.set_heads(attached_heads)
 
         head_state_dict = {}
@@ -252,7 +252,7 @@ def load_head_states(self, state, strict=True):
             for head_name, head_state in head_states.items():
                 self._heads[block_name][head_name].load_state_dict(head_state, strict)
 
-    def set_classy_state(self, state, strict=True):
+    def load_state_dict(self, state, strict=True):
         """Set the state of the ClassyModel.
 
         Args:
@@ -270,11 +270,17 @@ def set_classy_state(self, state, strict=True):
         # fetched / set when there are no blocks attached.
         attached_heads = self.get_heads()
         self.clear_heads()
-        self.load_state_dict(state["model"]["trunk"], strict)
+        super().load_state_dict(state["model"]["trunk"], strict)
 
         # set the heads back again
         self.set_heads(attached_heads)
 
+    def get_classy_state(self, deep_copy=False):
+        return self.state_dict(deep_copy=deep_copy)
+
+    def set_classy_state(self, state, strict=True):
+        self.load_state_dict(state, strict=strict)
+
     def forward(self, x):
         """
         Perform computation of blocks in the order define in get_blocks.