Fix scalar constant check

[gramalingam]

Fix scalar constant check.

TODO: for some optimizations, generalizations are possible, but they must be done on a rule-by-rule basis. Eg., for eliminating an addition of zero to x: eliminating this is always safe zero is a scalar, but if it is multi-dimensional, then it is safe if its rank is less than that of x.

[codecov]

Codecov Report

❌ Patch coverage is 36.84211% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.43%. Comparing base (9b699ae) to head (6f5b379).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
onnxscript/rewriter/models/_smollm_2.py	0.00%	7 Missing ⚠️
...script/rewriter/models/_rotary_embedding_models.py	0.00%	6 Missing ⚠️
onnxscript/rewriter/models/_smollm_1.py	0.00%	5 Missing ⚠️
onnxscript/rewriter/ort_fusions/gqa_test.py	0.00%	4 Missing ⚠️
onnxscript/rewriter/_matcher.py	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2672   +/-   ##
=======================================
  Coverage   70.43%   70.43%           
=======================================
  Files         224      224           
  Lines       26771    26778    +7     
  Branches     2680     2680           
=======================================
+ Hits        18855    18862    +7     
  Misses       6985     6985           
  Partials      931      931           

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