CoCa v2: fixes and improvements #554

There has been some issues raised about some mistakes made in the current CoCa implementation and there are also some improvements that can be made. This issue will enumerate them and we can track progress here. Once the TODO's are completed we can attempt another set of re-training runs and maybe even scale if the results make sense.

Problems:

• MultimodalTransformer initialization (CoCa: fix MultimodalTransformer init + Mask CLS token at end of seq #551)
• Mask CLS token at end of sequence (CoCa: fix MultimodalTransformer init + Mask CLS token at end of seq #551)
• Number of tokens problem (Issue with number of tokens for CoCa #458)
• Fix pooling

Improvements:

• Integrate generation with HF (so we can take out generate func)

Please add anything that I might've missed
cc: @gpucce @rom1504 @rwightman