consistent checkpoint filenaming (#865)

* consistent checkpoint filenaming

* backward compatible rename
Co-authored-by: NOlatunji Ruwase <olruwase@microsoft.com>

deepspeed/runtime/engine.py

@@ -1352,7 +1352,7 @@ class DeepSpeedEngine(Module):
         zero_ckpt_name = os.path.join(
             checkpoints_path,
             str(tag),
-            filename + '_mp_rank_{:02d}'.format(mp_rank) + 'optim_states.pt')
+            filename + '_mp_rank_{:02d}'.format(mp_rank) + '_optim_states.pt')
         return zero_ckpt_name
 
     def _get_zero_ckpt_name(self, checkpoints_path, tag):
@@ -1529,13 +1529,20 @@ class DeepSpeedEngine(Module):
             mp_rank=mp_rank,
             dp_world_size=self.loaded_checkpoint_dp_world_size)
         invalid_zero_ckpt_paths = []
-        for ckpt_name in zero_ckpt_names:
+        for i, ckpt_name in enumerate(zero_ckpt_names):
             if not os.path.exists(ckpt_name):
+                # transparently handle the old file pattern for optim_states
+                if 'optim_states.pt' in ckpt_name:
+                    ckpt_name_try = ckpt_name.replace("_optim_states.pt",
+                                                      "optim_states.pt")
+                    if os.path.exists(ckpt_name_try):
+                        zero_ckpt_names[i] = ckpt_name_try
+                        continue
                 invalid_zero_ckpt_paths.append(ckpt_name)
 
         if len(invalid_zero_ckpt_paths) > 0:
             logger.warn(
-                f"Client provided zero checkpoint load paths: {invalid_zero_ckpt_paths} does not exist"
+                f"The following zero checkpoints paths are missing: {invalid_zero_ckpt_paths}"
             )
             return None