Attention with caching #429
                
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Summary:
as title
adding attention with F.scaled_dot_product_attention, this class can be used for both self-attention and cross-attention.
details in https://docs.google.com/document/d/1a32JYAmXuwNX4c9HaZmrP1lrKlR_3A1skmzW6uAE7iM/edit#heading=h.vgbzma4xcyfm
Reviewed By: ankitade
Differential Revision: D46377616