[sdpa decomp] change sdpa decomp to be consistent with flash attention (#108608)
Summary: See the comment in code for the reasons of the change
Test Plan:
buck2 test executorch/examples/export/test:test_export --
test_vit_export_to_executorch
Differential Revision: D48992180
Pull Request resolved: https://github.com/pytorch/pytorch/pull/108608
Approved by: https://github.com/larryliu0820
