[torchlib] Fix concat when input tensor has shape (0,)

[justinchuby]

Filter out size-0 tensors. When there is only one input, create an identity op instead of a concat op.

Fix #2660

[codecov]

Codecov Report

❌ Patch coverage is 63.63636% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.41%. Comparing base (69025f7) to head (a421f57).
⚠️ 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	63.63%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2661      +/-   ##
==========================================
- Coverage   70.41%   70.41%   -0.01%     
==========================================
  Files         224      224              
  Lines       26591    26600       +9     
  Branches     2641     2645       +4     
==========================================
+ Hits        18725    18730       +5     
- Misses       6943     6945       +2     
- Partials      923      925       +2     

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