Implement SDPA via MHA

[gramalingam]

Implement SDPA via MHA. This handles the case when earlier fusion rules do not map larger patterns containing SDPA into MHA or GQA or Attention (from ORT contrib ops). It implements SDPA via MHA.

[codecov]

Codecov Report

❌ Patch coverage is 60.60606% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.11%. Comparing base (d80575d) to head (aeee0fa).
⚠️ Report is 5 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
onnxscript/rewriter/ort_fusions/sdpa_test.py	42.10%	11 Missing ⚠️
onnxscript/rewriter/ort_fusions/sdpa_via_mha.py	83.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2683   +/-   ##
=======================================
  Coverage   70.11%   70.11%           
=======================================
  Files         224      225    +1     
  Lines       26982    27068   +86     
  Branches     2705     2721   +16     
=======================================
+ Hits        18919    18980   +61     
- Misses       7129     7148   +19     
- Partials      934      940    +6     

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