[AI Accelerators] Update torch.nn.functional multi_head_attention_forward(). Add pastpath

[ynonaolga]

Summary: It presents as a fastpath where common execution paths are executed natively in Pytorch using torch._native_multi_head_attention. At present, the fast path is implemented in torch.nn.MultiHeadAttention.
Change replicates fastpath implementation in torch.nn.functional multi_head_attention_forward.
With some refactoring done to avoid duplication.

Test Plan: buck2 test @//mode/dev-nosan //caffe2/test:jit -- 'jit_multihead_attn_forward (test_jit.TestJit)' test_functional_multi_head_attn_fast_path --print-passing-details

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/88912

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[mikekgfb]

What is this change? Is this to avoid the assertion that you added elsewhere to prevent bias from being None?

Projection bias can be None, see the test and fix in this as per issue raised. #88970

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