Add grayscale transform for videos

[lyaronskaya]

Motivation and Context

The grayscale transformation for video is used in video action classification approaches to accelerate training.

How Has This Been Tested

The result of grayscale transform is compared to frame-wise torchvision.transforms.grayscale transformation.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.