Add support for backtracking in pattern matcher (#2273)

Extends the recently introduced "Or" patterns fully (without any
restrictions). The general case is handled via backtracking. The Or
pattern constructor function will automatically determine whether the
optimized (deterministic) matching implementation can be used or if the
backtracking-based implementation should be used.

Author: gramalingam

onnxscript/rewriter/generic_pattern.py

@@ -83,7 +83,9 @@ def _to_match_result(pmr: PatternMatchResult) -> orp.MatchResult:
     TODO: This is a temporary hack until MatchResult and PatternMatchResult are unified.
     """
     result = orp.MatchResult()
-    result.nodes.extend(pmr.model_nodes)
+    for node in pmr.model_nodes:
+        result.add_node(node)
+
     for var, val in pmr.matched_pattern_to_model_value.items():
         if var.name is not None:
             result.bind(var.name, val)