Add CrossAttention operator (#14146)
Move separated Q, K and V (without input projection) from Attention to a
new operator CrossAttention.
The Attention operator is hard to maintain when we need support with and
without input projection in one class. Add a new operator according to
feedback.
Some change might need in the future, but not in this PR:
(1) bias could be optional (We will not proceed that route unless
experiments show that fusing Add bias with MatMul instead of this op
could improve performance).
(2) support packed KV. There are two ways to support it: when key and
value are same Tensor, they are packed; or we can make value as
optional, and use packed mode when value is empty and the key has packed
K/V.
(3) support cached key and value, and other (like relative position
bias), or more attention mask format. They can be added easily without
breaking backward compatible.
(4) ROCm/CPU implementation of this op.
