Video clips workers (#1369)

* Expose num_workers in VideoClips

* add documentation for num_workers in VideoClips

* add documentation for num_workers in VideoClips

* add documentation for num_workers in VideoClips

Author: ekosman

torchvision/datasets/video_utils.py

@@ -64,13 +64,15 @@ class VideoClips(object):
         frame_rate (int, optional): if specified, it will resample the video
             so that it has `frame_rate`, and then the clips will be defined
             on the resampled video
+        num_workers (int): how many subprocesses to use for data loading.
+            0 means that the data will be loaded in the main process. (default: 0)
     """
     def __init__(self, video_paths, clip_length_in_frames=16, frames_between_clips=1,
-                 frame_rate=None, _precomputed_metadata=None, num_workers=1,
-                 _backend="pyav"):
+                 frame_rate=None, _precomputed_metadata=None, num_workers=0, _backend="pyav"):
         self.video_paths = video_paths
         self.num_workers = num_workers
         self._backend = _backend
+
         if _precomputed_metadata is None:
             self._compute_frame_pts()
         else: