[torchlib] Fix operator add

[justinchuby]

Operator add may take in python scalar sometimes (and doesn't have dtype). This PR splits the implementation out so that it is handled differently.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.39%. Comparing base (32a61f4) to head (d2f08bc).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
onnxscript/function_libs/torch_lib/ops/core.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2630      +/-   ##
==========================================
- Coverage   70.39%   70.39%   -0.01%     
==========================================
  Files         222      222              
  Lines       26275    26281       +6     
  Branches     2629     2629              
==========================================
+ Hits        18496    18500       +4     
- Misses       6859     6861       +2     
  Partials      920      920              

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