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 70.12%. Comparing base (d80575d) to head (21a1594).
⚠️ Report is 2 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%   70.12%   +0.01%     
==========================================
  Files         224      225       +1     
  Lines       26982    27063      +81     
  Branches     2705     2719      +14     
==========================================
+ Hits        18919    18979      +60     
- Misses       7129     7146      +17     
- Partials      934      938       +4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[justinchuby]

Create a docstring for these params?

[gramalingam]

Thinking ... [ok, I picked that from chatgpt :-)] ... I suspect what you are asking for is a "spec" for the SDPA (the internal op used by fusion, which does not correspond to any ONNX standard op or ORT contrib op, though it is a close approximation to ONNX's Attention). Is that right? Because that op is used across multiple fusions and files, the sdpa.py file might be the right place for that. Because the 1-liner docstring for these params would be that they correspond to the corresponding inputs and attribute of the SDPA op ... which doesn't say anything more than what the reader can see in the next few lines.

[gramalingam]

Ok, added documentation as discussed above.