[ViT_MAE] fix num of channels in `patchify` and `unpatchify`
# What does this PR do?
Fix the hard-coded number of channels in `patchify` and `unpatchify` methods.

Fixes #17473 

## Who can review?
@NielsRogge 

_The documentation is not available anymore as the PR was closed or merged._<|||||>@NielsRogge I didn't find what is breaking the doc now<|||||>@NielsRogge , can you re-run the gh action? looks like something interrupted it<|||||>Hi,

I've created PR #17710 that fixes some more things, like the variable names and docstrings.