Add random temporal subsample transform

[davidemaz]

Motivation and Context

This add a random temporal subsample transform. Currently just the uniform temporal subsample is implemented.
It's a data-augmentation strategy to be used during training to increase the dataset variability.
The model sees different frames from the same video clip at every epoch.

How Has This Been Tested

Unit tests plus training a model with this augmentation function.

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.