Make SeqRS invokes checkpoint/restore only when they are available (#1008)

Summary:
Per title

Pull Request resolved: #1008

Reviewed By: mingyuzh

Differential Revision: D43243392

Pulled By: ejguan

fbshipit-source-id: cad70b7a859481bc293a95e45979c98b589bb90a

Author: ejguan

torchdata/dataloader2/reading_service.py

@@ -6,6 +6,7 @@
 
 import multiprocessing as py_mp
 import queue
+import warnings
 
 from abc import ABC, abstractmethod
 from datetime import timedelta
@@ -534,13 +535,20 @@ def finalize_iteration(self) -> None:
     def checkpoint(self) -> bytes:
         states = []
         for rs in self.reading_services:
-            states.append(rs.checkpoint())
+            if hasattr(rs, "checkpoint") and callable(rs.checkpoint):
+                states.append(rs.checkpoint())
+            else:
+                warnings.warn(f"{rs} doesn't support `checkpoint`, skipping...")
+                states.append(b"")
         return b"\n".join(states)
 
     # Sequential Order, to align with initialize
     def restore(self, datapipe, serialized_state: bytes) -> DataPipe:
         states = serialized_state.split(b"\n")
         assert len(states) == len(self.reading_services)
         for rs, state in zip(self.reading_services, states):
-            datapipe = rs.restore(datapipe, state)
+            if hasattr(rs, "restore") and callable(rs.restore):
+                datapipe = rs.restore(datapipe, state)
+            else:
+                warnings.warn(f"{rs} doesn't support `restore` from state, skipping...")
         return datapipe