Refactor rewrite rules into the rewriter.rules namespace

[justinchuby]

Organize all rules into a directory that is not with the rewriter infrastructure:

• onnxscript.rewriter.rules.common.* for existing rules
• onnxscript.rewriter.rules.fusion.* for onnx fusion rules

[codecov]

Codecov Report

❌ Patch coverage is 99.23077% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 69.91%. Comparing base (2cc2502) to head (715f975).
⚠️ Report is 5 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...xscript/rewriter/rules/common/_basic_rules_test.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2531      +/-   ##
==========================================
- Coverage   70.00%   69.91%   -0.09%     
==========================================
  Files         215      216       +1     
  Lines       25988    26049      +61     
  Branches     2606     2614       +8     
==========================================
+ Hits        18192    18213      +21     
- Misses       6896     6936      +40     
  Partials      900      900              

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[gramalingam]

Thanks for doing this, have been thinking about doing this for a while!

One question: do we need a separate folder, or should this be together with onnx_fusions? There is a clear distinction between ort_fusions and onnx_fusions: but for rules/optimizations that apply to standard onnx, what would the distinction between onnx_fusions and rules be?

[justinchuby]

@gramalingam updated PR description and commits. For now I didn't touch ort_fusions. For onnx fusions, I organized the rules into rules/fusion.

[justinchuby]

@gramalingam PTAL