Implement einsum | feat(torchlib)

[justinchuby]

Implement einsum and fix a bug in the test code where we mistakenly treated str as a tensor. xfailed cases related to onnx/onnx#5739 and when input is int32.

Fixes #1130

[codecov]

Codecov Report

Merging #1133 (e139b1c) into main (9c5770b) will increase coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head e139b1c differs from pull request most recent head 011c357. Consider uploading reports for the commit 011c357 to get more accurate results

@@            Coverage Diff             @@
##             main    #1133      +/-   ##
==========================================
+ Coverage   78.38%   78.40%   +0.01%     
==========================================
  Files         118      118              
  Lines       15030    15033       +3     
  Branches     1601     1601              
==========================================
+ Hits        11782    11786       +4     
+ Misses       2878     2877       -1     
  Partials      370      370              

Files	Coverage Δ
onnxscript/function_libs/torch_lib/ops/core.py	79.78% <100.00%> (+0.03%)	⬆️
...t/tests/function_libs/torch_lib/ops_test_common.py	86.07% <100.00%> (ø)
...ipt/tests/function_libs/torch_lib/ops_test_data.py	96.18% <100.00%> (+0.02%)	⬆️