[torchlib] Fix aten_gather to correctly handle scalar indices

[linshokaku]

Fixes #2564

[codecov]

Codecov Report

❌ Patch coverage is 20.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.32%. Comparing base (f529292) to head (57cbb4e).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
onnxscript/function_libs/torch_lib/ops/core.py	20.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2566      +/-   ##
==========================================
- Coverage   70.33%   70.32%   -0.01%     
==========================================
  Files         222      222              
  Lines       26656    26659       +3     
  Branches     2666     2667       +1     
==========================================
+ Hits        18748    18749       +1     
- Misses       6992     6993       +1     
- Partials      916      917       +1     

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