[ViTMAE] Fix docstrings and variable names
# What does this PR do?

Fixes #17473
Fixes #17665

This PR improves the docstrings and variable names of the patchify, unpatchify and forward_loss methods of ViTMAE. This way, also the number of channels isn't hardcoded anymore.

cc @sayakpaul 
_The documentation is not available anymore as the PR was closed or merged._