Implement __repr__ for MatchResult (microsoft#2353)

This pull request adds a `__repr__` method to the `MatchResult` class in
`onnxscript/rewriter/_basics.py`. The new method provides a string
representation of the match result, improving debugging and readability.

Author: justinchuby

onnxscript/rewriter/_basics.py

@@ -38,6 +38,14 @@ def __init__(self) -> None:
         # We use a stack of partial matches to handle OR patterns that require backtracking.
         self._partial_matches: list[PartialMatchResult] = [PartialMatchResult()]
 
+    def __repr__(self) -> str:
+        """Returns a string representation of the match result."""
+        if not self._partial_matches:
+            return "MatchResult()"
+        return (
+            f"MatchResult(success={bool(self)}, reason={self.reason!r}, nodes={self.nodes!r})"
+        )
+
     @property
     def _current_match(self) -> PartialMatchResult:
         """Returns the current match result."""