fix(nyz): fix offline data fetcher bugs

ding/framework/middleware/functional/data_processor.py

@@ -239,6 +239,7 @@ def offline_data_fetcher(cfg: EasyDict, dataset: Dataset) -> Callable:
     """
     # collate_fn is executed in policy now
     dataloader = DataLoader(dataset, batch_size=cfg.policy.learn.batch_size, shuffle=True, collate_fn=lambda x: x)
+    dataloader = iter(dataloader)
 
     def _fetch(ctx: "OfflineRLContext"):
         """
@@ -250,10 +251,17 @@ def _fetch(ctx: "OfflineRLContext"):
         Output of ctx:
             - train_data (:obj:`List[Tensor]`): The fetched data batch.
         """
-        while True:
-            for i, data in enumerate(dataloader):
-                ctx.train_data = data
-                yield
+        nonlocal dataloader
+        try:
+            ctx.train_data = next(dataloader)  # noqa
+        except StopIteration:
+            ctx.train_epoch += 1
+            del dataloader
+            dataloader = DataLoader(
+                dataset, batch_size=cfg.policy.learn.batch_size, shuffle=True, collate_fn=lambda x: x
+            )
+            dataloader = iter(dataloader)
+            ctx.train_data = next(dataloader)
         # TODO apply data update (e.g. priority) in offline setting when necessary
 
     return _fetch