Utility and example for custom op expansion

[gramalingam]

A utility and an example showing how onnxscript functions can be used to define function expansions and be used with the inliner to replace calls to the custom function with an expanded subgraph. This is useful to perform certain classes of graph surgery easily.

[titaiwangms]

I will merge this for now.

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.04%. Comparing base (97513c7) to head (a98aaad).
⚠️ Report is 14 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
onnxscript/utils/replace.py	33.33%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2701      +/-   ##
==========================================
- Coverage   70.06%   70.04%   -0.03%     
==========================================
  Files         225      226       +1     
  Lines       27162    27177      +15     
  Branches     2732     2734       +2     
==========================================
+ Hits        19031    19036       +5     
- Misses       7187     7197      +10     
  Partials      944      944              

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