Prevent in_memory_cache from yielding from source_dp when it's fu…

…lly cached (#1160)

Summary:
Fixes #1159.

Pull Request resolved: #1160

Reviewed By: NivekT

Differential Revision: D45883150

Pulled By: ejguan

fbshipit-source-id: 8aa15294bf32559334817a1b8ec8b88b2807fd5a

torchdata/datapipes/iter/util/cacheholder.py

@@ -97,11 +97,12 @@ def __init__(self, source_dp: IterDataPipe[T_co], size: Optional[int] = None) ->
 
     def __iter__(self) -> Iterator[T_co]:
         if self.cache:
-            for idx, data in enumerate(self.source_dp):
-                if idx < self.idx:
-                    yield data
-                else:
-                    break
+            if self.idx > 0:
+                for idx, data in enumerate(self.source_dp):
+                    if idx < self.idx:
+                        yield data
+                    else:
+                        break
             yield from self.cache
         else:
             # Local cache