[ViTMAE] Renamed variable name
The `sequence_masked` variable is actually the part of the sequence that is kept **unmasked** for the encoder to consume. This commit renames the variable accordingly.

CC: @sayakpaul 
_The documentation is not available anymore as the PR was closed or merged._<|||||>Tagging @NielsRogge, as he's the vision master 👨‍🏫 <|||||>Just a reminder here!<|||||>(Niels it currently off, he'll be back in a few days :) )<|||||>FYI, I took this from the original repository: https://github.com/facebookresearch/mae/blob/efb2a8062c206524e35e47d04501ed4f544c0ae8/models_mae.py#L140<|||||>Do you think an issue in the original repository would be a good approach to move forward? @NielsRogge <|||||>Yes indeed, to make the authors confirm!<|||||>Thanks for leading the effort. Good work! 

@sgugger and @NielsRogge thanks for the reviews and actions. 