[pytorch] Disable fast path in MultiheadAttention in Export (#106824)
Summary:
We are seeing `aten._native_multi_head_attention` op (not in core Aten op set) is left in the exported graph and causes problems in the downstream at runtime.
Two proposed solutions:
1. Disable fast path while tracing to leverage the non-optimized path to get decomp, that way, the blamed op won't show up in the exported graph
2. Add a decomp rule for `aten._native_multi_head_attention`
After discussing with kimishpatel and bdhirsh, #1 is preferred and verified it could immediately unblock the critical model enablement work for PP.
Test Plan: CI
Differential Revision: D48169806
Pull Request resolved: https://github.com/pytorch/pytorch/pull/106824
Approved by: https://github.com/kimishpatel
