add support for creating LabeledVideoDataset from video frames

[fatih c. akyon]

Currently, Video object can be constructed from a video folder that contains video frames:

pytorchvideo/pytorchvideo/data/frame_video.py

Line 115 in 08a1fce

 def from_frame_paths( 

However, LabeledVideoDataset cannot be created from video frames since LabeledVideoPaths class does not support that:

pytorchvideo/pytorchvideo/data/labeled_video_dataset.py

Line 296 in 08a1fce

 labeled_video_paths = LabeledVideoPaths.from_path(data_path) 

To solve this issue, from_path method of the LabeledVideoPaths

pytorchvideo/pytorchvideo/data/labeled_video_paths.py

Line 19 in 08a1fce

 def from_path(cls, data_path: str) -> LabeledVideoPaths: 

can be extended so that it can handle video frames.

[kalyanvasudev]

Thanks for working this! I will review the PR soon.