fix(atenlib): combine cross_entropy_loss

[justinchuby]

• Fix static optional support in ParamSchema
• Make cross_entropy_loss a script function

Follow up of https://github.com/microsoft/onnx-script/pull/444/files#diff-647f5c6f5a51c850e0a6bfd534cc8d1357d5b617c432693af121830b65a324c2R242

[codecov]

Codecov Report

Merging #555 (b1b7e0b) into main (9fe15b2) will increase coverage by 0.01%.
The diff coverage is 42.85%.

@@            Coverage Diff             @@
##             main     #555      +/-   ##
==========================================
+ Coverage   73.91%   73.92%   +0.01%     
==========================================
  Files         109      109              
  Lines       11007    11004       -3     
  Branches     1142     1142              
==========================================
- Hits         8136     8135       -1     
+ Misses       2564     2561       -3     
- Partials      307      308       +1     

Impacted Files	Coverage Δ
...s/function_libs/torch_aten/ops_correctness_test.py	90.27% <ø> (ø)
onnxscript/function_libs/torch_aten/ops/nn.py	66.57% <33.33%> (+0.52%)	⬆️
onnxscript/values.py	91.70% <50.00%> (-0.93%)	⬇️

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