Add on_eval_epoch_end as a valid hook to TorchSnapshotSaver (#826)

Summary:

When `TorchSnapshotSaver` is used with `save_every_n_eval_epochs > 0` and `best_checkpoint_config`, this hook is invoked.

https://www.internalfb.com/code/fbsource/[a8a4a7fba9a8a93af7382fa12e669c066f41024f]/fbcode/torchtnt/framework/callbacks/base_checkpointer.py?lines=273

However, it fails due to not being considered a valid hook. This diff fixes that

Differential Revision: D57083777

Author: dorukhansergin

torchtnt/framework/callbacks/torchsnapshot_saver.py

@@ -157,7 +157,12 @@ def _checkpoint_impl(
         """
         Checkpoint the current state of the application.
         """
-        if hook not in ["on_train_step_end", "on_train_epoch_end", "on_train_end"]:
+        if hook not in [
+            "on_train_step_end",
+            "on_train_epoch_end",
+            "on_train_end",
+            "on_eval_epoch_end",
+        ]:
             raise RuntimeError(f"Unexpected hook encountered '{hook}'")
 
         intra_epoch = False