Bug fix with prefetch_factor

Summary:
Fix #5086.
The PyTorch DataLoader class can accepts "None" as prefetch_factor in recent versions (> 2.0). For backward compatibility, it is better to set a default value, specifically 2 as in previous PyTorch versions.
Looking at the more recent DataLoader source code, it sets the value 2 if prefetch_factor is found to be None.

Pull Request resolved: #5091

Reviewed By: wat3rBro

Differential Revision: D50693761

Pulled By: ezyang

fbshipit-source-id: 479ec794009be9e95d27c401143a88dcd45a6eff

detectron2/data/build.py

@@ -300,9 +300,7 @@ def build_batch_data_loader(
     num_workers=0,
     collate_fn=None,
     drop_last: bool = True,
-    prefetch_factor=None,
-    persistent_workers=False,
-    pin_memory=False,
+    **kwargs,
 ):
     """
     Build a batched dataloader. The main differences from `torch.utils.data.DataLoader` are:
@@ -341,9 +339,7 @@ def build_batch_data_loader(
             num_workers=num_workers,
             collate_fn=operator.itemgetter(0),  # don't batch, but yield individual elements
             worker_init_fn=worker_init_reset_seed,
-            prefetch_factor=prefetch_factor,
-            persistent_workers=persistent_workers,
-            pin_memory=pin_memory,
+            **kwargs
         )  # yield individual mapped dict
         data_loader = AspectRatioGroupedDataset(data_loader, batch_size)
         if collate_fn is None:
@@ -357,9 +353,7 @@ def build_batch_data_loader(
             num_workers=num_workers,
             collate_fn=trivial_batch_collator if collate_fn is None else collate_fn,
             worker_init_fn=worker_init_reset_seed,
-            prefetch_factor=prefetch_factor,
-            persistent_workers=persistent_workers,
-            pin_memory=pin_memory,
+            **kwargs
         )
 
 
@@ -499,9 +493,7 @@ def build_detection_train_loader(
     aspect_ratio_grouping=True,
     num_workers=0,
     collate_fn=None,
-    prefetch_factor=None,
-    persistent_workers=False,
-    pin_memory=False,
+    **kwargs
 ):
     """
     Build a dataloader for object detection with some default features.
@@ -553,9 +545,7 @@ def build_detection_train_loader(
         aspect_ratio_grouping=aspect_ratio_grouping,
         num_workers=num_workers,
         collate_fn=collate_fn,
-        prefetch_factor=prefetch_factor,
-        persistent_workers=persistent_workers,
-        pin_memory=pin_memory,
+        **kwargs
     )