Adding MaskFormer

[vedantdalimkar]

MaskFormer is the precursor to Mask2Former and since there are plans to add Mask2Former (#1044), I thought it would be natural to have a MaskFormer implementation first so that it's components can facilitate a more quicker development of Mask2Former. I would be happy to contribute to this issue if it's fine.

[qubvel]

Hey @vedantdalimkarm thanks for opening the issue! To minimize the maintained code, I think we should add V2 rather than V1, cause it's a newer version

[vedantdalimkar]

Hey @vedantdalimkarm thanks for opening the issue! To minimize the maintained code, I think we should add V2 rather than V1, cause it's a newer version

Sure, that makes sense. Will start working on Mask2Former implementation directly then.