[ViTMAE] Add copied from statements and fix prefix
# What does this PR do?

Fixes #16101 by updating `base_model_prefix` from "vit_mae" to "vit" (`ViTMAEForPreTraining` is using the latter).

Also adds copied from statements to ensure consistency with `modeling_vit.py`.
_The documentation is not available anymore as the PR was closed or merged._