Ensure exporter creates valid python name for opset vars

[gramalingam]

The existing exporter creates invalid python identifiers like "onnxscript.atenlib1" when generating variable names for opsets. Change this to create a name like "onnxscript_atenlib1".

[codecov]

Codecov Report

Merging #1051 (4a6a399) into main (8af8161) will increase coverage by 0.02%.
The diff coverage is 82.22%.

@@            Coverage Diff             @@
##             main    #1051      +/-   ##
==========================================
+ Coverage   77.75%   77.78%   +0.02%     
==========================================
  Files         114      114              
  Lines       14489    14517      +28     
  Branches     1549     1554       +5     
==========================================
+ Hits        11266    11292      +26     
- Misses       2857     2858       +1     
- Partials      366      367       +1     

Files Changed	Coverage Δ
onnxscript/backend/onnx_export.py	70.12% <79.41%> (+1.61%)	⬆️
onnxscript/backend/onnx_export_test.py	86.04% <90.90%> (+0.09%)	⬆️

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[github-actions]

Test Results

         18 files  ±0         18 suites  ±0   1h 11m 33s ⏱️ - 8m 35s
  10 616 tests +1    7 879 ✔️ +1      2 737 💤 ±0  0 ❌ ±0 
150 633 runs  +9  33 263 ✔️ +9  117 370 💤 ±0  0 ❌ ±0 

Results for commit 4a6a399. ± Comparison against base commit 8af8161.

♻️ This comment has been updated with latest results.