Add "latest_checkpoint" path to state_dict (#169)

Doesn't quite mirror [this cog
PR](https://github.com/EmbarkStudios/cog/pull/731/files), but ensures we
have the path, as discussed with @tgolsson. Please let me know in case I
misunderstood something.

emote/callbacks/checkpointing.py

@@ -59,19 +59,25 @@ def begin_training(self):
         os.makedirs(self._folder_path, exist_ok=True)
 
     def end_cycle(self, bp_step, bp_samples):
+        name = f"checkpoint_{self._checkpoint_index}.tar"
+        final_path = os.path.join(self._folder_path, name)
         state_dict = {
             "callback_state_dicts": {cb.name: cb.state_dict() for cb in self._cbs},
             "training_state": {
+                "latest_checkpoint": final_path,
                 "bp_step": bp_step,
                 "bp_samples": bp_samples,
                 "checkpoint_index": self._checkpoint_index,
             },
         }
-        name = f"checkpoint_{self._checkpoint_index}.tar"
-        final_path = os.path.join(self._folder_path, name)
         torch.save(state_dict, final_path)
         self._checkpoint_index += 1
 
+        return {
+            "latest_checkpoint": state_dict["training_state"]["latest_checkpoint"],
+            "checkpoint_index": state_dict["training_state"]["checkpoint_index"],
+        }
+
 
 class CheckpointLoader(Callback):
     """CheckpointLoader loads a checkpoint like the one created by Checkpointer.

tests/test_checkpoints.py

@@ -70,6 +70,8 @@ def test_networks_checkpoint():
     t1.train()
     n2 = nn.Linear(1, 1)
     test_data = torch.rand(5, 1)
+
+    assert "latest_checkpoint" in t1.state
     assert not torch.allclose(n1(test_data), n2(test_data))
 
     c2 = [