[Flexible Outputs] Allow multi-output matching

[srikris-sridhar]

Adds support for pattern matching on multiple outputs for optimizations involving ops like Split.

def pattern(self, op, x):
        relu = op.Relu(x)
        return op.Split(relu, _allow_flexible_outputs=True)

Fixes #2581

[srikris-sridhar]

@justinchuby Hopefully this is a good attempt. It's my first PR, so go easy on me :)

[codecov]

Codecov Report

❌ Patch coverage is 88.37209% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.98%. Comparing base (1077da7) to head (2d0cc42).

Files with missing lines	Patch %	Lines
onnxscript/rewriter/pattern_test.py	82.60%	2 Missing and 2 partials ⚠️
onnxscript/rewriter/_rewrite_rule.py	92.85%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2873      +/-   ##
==========================================
+ Coverage   71.96%   71.98%   +0.02%     
==========================================
  Files         239      239              
  Lines       29224    29262      +38     
  Branches     2878     2885       +7     
==========================================
+ Hits        21031    21065      +34     
- Misses       7216     7218       +2     
- Partials      977      979       +2     

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[srikris-sridhar]

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