refactor hard coded numbers for more control over parameters (MaskedAutoencoderConvViT)

[DanTaranis]

Hi - I'd like to do patches of size 32x32, and a smaller model in general. any thing I change breaks the entire code. It would be really helpful if you refactored out all of the places that specify 4,2,16...etc throughout the code for MaskedAutoencoderConvViT

Thanks, Dan