Implement a masking strategy which masks consistently across temporal frames.

[kdu4108]

i.e., if you mask out patch position 7 for one frame, do it for all frames in that clip. The motivation here is that if you randomly mask patches then the model can probably "cheat" to predict the patch from the adjacent frames, as discussed with @garjania.

def of done (DoD): implement this masking strategy, ideally test it too