[NVIDIA XLA:GPU] Fused MHA Support in XLA GPU: SE
@akuegel @cheshire Based on https://github.com/tensorflow/tensorflow/pull/60139#issuecomment-1486799319, I created this new PR. @akuegel Possible to merge this now? Hopefully no more issues :)
cuDNN currently supports the following patterns for Multi-headed attention:

BMM1 - BMM2
BMM1 - Scale - Bias - Mask - Softmax - BMM2
BMM1 - Scale - Bias - Mask - Softmax - Dropout - BMM2
BMM1 - Scale - Mask - Softmax - BMM2
BMM1 - Scale - Mask - Softmax - Dropout - BMM2
BMM1 - Softmax - Dropout - BMM2

This PR adds support for the stream executor for these patterns.