[Flexible Outputs] Allow multi-output matching

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

Author: srikris-sridhar

onnxscript/rewriter/_matcher.py

@@ -310,9 +310,12 @@ def _match_single_output_node(
         if output_values is None:
             # TODO(rama): Is this a valid (useful) case?
             return match
-        if check_removable and not _valid_to_replace(match.nodes, output_values):
-            # TODO(rama): Match status should be updated to reflect failure reason.
-            return match.fail("Matched nodes have other uses preventing replacement.")
+        # Skip removability check for flexible output nodes since they may have
+        # additional outputs beyond those captured in the pattern
+        if check_removable and not pattern.output_node.allow_flexible_outputs:
+            if not _valid_to_replace(match.nodes, output_values):
+                # TODO(rama): Match status should be updated to reflect failure reason.
+                return match.fail("Matched nodes have other uses preventing replacement.")
 
         match.outputs.extend(output_values)
         return match

onnxscript/rewriter/_pattern_ir.py

@@ -248,6 +248,7 @@ def __call__(
         _outputs: int | list[str | None] = 1,
         _allow_other_attributes: bool | None = None,
         _allow_other_inputs: bool | None = None,
+        _allow_flexible_outputs: bool | None = None,
         _check: Callable | None = None,
         **kwargs,
     ):
@@ -280,6 +281,7 @@ def __call__(
             _outputs,
             allow_other_attributes=_allow_other_attributes,
             allow_other_inputs=_allow_other_inputs,
+            allow_flexible_outputs=_allow_flexible_outputs,
             check=_check,
         )
         self.pattern_builder.add_node(node_pattern)
@@ -440,6 +442,7 @@ def __init__(
         *,
         allow_other_attributes: bool | None,
         allow_other_inputs: bool | None,
+        allow_flexible_outputs: bool | None = None,
         check: Callable | None = None,
     ):
         if allow_other_attributes is None:
@@ -448,12 +451,16 @@ def __init__(
         if allow_other_inputs is None:
             # TODO(rama): Should we default to True? For now, we preserve the current behavior.
             allow_other_inputs = False
+        if allow_flexible_outputs is None:
+            # Default behavior: do not match flexible outputs 
+            allow_flexible_outputs = False
         self.domain = domain
         self.op = StringConstantPattern(op) if isinstance(op, str) else op
         self.inputs = [_to_value_pattern(x) for x in inputs]
         self.attributes = attributes
         self.allow_other_attributes = allow_other_attributes
         self.allow_other_inputs = allow_other_inputs
+        self.allow_flexible_outputs = allow_flexible_outputs
         self._check = check
         # In the common case, domain and op are constants, which can be used to optimize matching.
         if isinstance(op, str) and isinstance(domain, StringConstantPattern):

onnxscript/rewriter/_rewrite_rule.py

@@ -6,6 +6,7 @@
 
 import abc
 import dataclasses
+import inspect
 import itertools
 from typing import (
     Callable,
@@ -214,11 +215,14 @@ class ReplacementPatternFunction:
 
     def __init__(self, function) -> None:
         self._function = function
+        # Cache signature inspection to avoid repeated introspection on hot path
+        self._accepts_match = "_match" in inspect.signature(function).parameters
 
     def get_replacement(self, match: _basics.MatchResult) -> ReplacementSubgraph | None:
         context = RewriterContext()
+        bindings = match.bindings if not self._accepts_match else {**match.bindings, "_match": match}
         try:
-            new_outputs = self._function(context, **match.bindings)
+            new_outputs = self._function(context, **bindings)
         except _basics.MatchFailureError as e:
             match.fail(e.reason, list(e.failure_sources))
             return None
@@ -313,6 +317,11 @@ def __init__(
         # Initialize the base pattern matching functionality
         super().__init__(target_pattern, condition_function, matcher, verbose, name)
 
+        # Check if any node in the pattern uses flexible outputs (cache for hot path)
+        self._has_flexible_outputs = any(
+            node.allow_flexible_outputs for node in self._target_pattern._nodes
+        )
+
         if not isinstance(replacement_pattern, ReplacementPatternFunction):
             replacement_pattern = ReplacementPatternFunction(replacement_pattern)
         self._replacement_pattern = replacement_pattern
@@ -357,7 +366,8 @@ def try_rewrite(
                     _basics.MatchStatus.REPLACEMENT_FAILED,
                 )
             return None
-        if len(replacement_subgraph.new_outputs) != self._target_pattern.num_outputs:
+
+        if not self._has_flexible_outputs and len(replacement_subgraph.new_outputs) != self._target_pattern.num_outputs:
             raise ValueError(
                 f"Number of outputs from replacement function does not match the number of outputs from the target pattern. "
                 f"Expected {self._target_pattern.num_outputs}, but got {len(replacement_subgraph.new_outputs)}."
@@ -766,14 +776,33 @@ def _apply_to_graph_or_function(
                     for n in delta.new_nodes:
                         n.metadata_props[RULE_NAME_TAG] = rule.name
 
-                convenience.replace_nodes_and_values(
-                    graph_or_function,
-                    node,
-                    delta.match.nodes if rule.remove_nodes else [],
-                    delta.new_nodes,
-                    delta.match.outputs,
-                    delta.new_outputs,
-                )
+                # Check if this is a flexible output case (matched node has more outputs than captured)
+                flexible_node = None
+                for matched_node in delta.match.nodes:
+                    if len(matched_node.outputs) > len(delta.match.outputs):
+                        flexible_node = matched_node
+                        break
+
+                if flexible_node and len(delta.new_outputs) == len(flexible_node.outputs):
+                    # Flexible output replacement: replace all outputs of the flexible node
+                    convenience.replace_nodes_and_values(
+                        graph_or_function,
+                        node,
+                        delta.match.nodes if rule.remove_nodes else [],
+                        delta.new_nodes,
+                        flexible_node.outputs,
+                        delta.new_outputs,
+                    )
+                else:
+                    # Standard replacement
+                    convenience.replace_nodes_and_values(
+                        graph_or_function,
+                        node,
+                        delta.match.nodes if rule.remove_nodes else [],
+                        delta.new_nodes,
+                        delta.match.outputs,
+                        delta.new_outputs,
+                    )
 
                 if merge_metadata:
                     _default_metadata_merger.copy_merged_metadata(

onnxscript/rewriter/pattern_test.py

@@ -989,5 +989,54 @@ def test_pattern_builder_context(self):
         self.assertEqual(ops, ["Op1", "Op2", "Add", "Op3", "Mul"])
 
 
+class FlexibleOutputTest(unittest.TestCase):
+    """Test patterns with flexible output counts using _allow_flexible_outputs."""
+
+    def test_flexible_outputs_with_split(self):
+        """Test that _allow_flexible_outputs works for Split with varying output counts."""
+
+        def relu_split_pattern(op, x):
+            relu = op.Relu(x)
+            return op.Split(relu, _allow_flexible_outputs=True)
+
+        def relu_split_rewrite(op, _match=None, x=None):
+            if x is None or _match is None:
+                return None
+
+            split = next((n for n in _match.nodes if n.op_type == "Split"), None)
+            if not split:
+                return None
+
+            num_outputs = len(split.outputs)
+            split_results = op.Split(x, _outputs=num_outputs, **split.attributes)
+
+            return tuple(op.Relu(s) for s in split_results) if num_outputs > 1 else op.Relu(split_results)
+
+        rule = pattern.RewriteRule(relu_split_pattern, relu_split_rewrite)
+
+        # Test model with Relu -> Split pattern
+        model_proto = onnx.parser.parse_model(
+            """
+            <ir_version: 7, opset_import: [ "" : 18]>
+            agraph (float[10] x) => (float[5] out1, float[5] out2)
+            {
+                relu_out = Relu(x)
+                out1, out2 = Split<axis=0, num_outputs=2>(relu_out)
+            }
+        """
+        )
+
+        optimized = onnxscript.rewriter.rewrite(model_proto, pattern_rewrite_rules=[rule])
+
+        # Verify transformation: 1 Relu + 1 Split -> 1 Split + 2 Relu
+        def count_ops(proto, op_type):
+            return sum(1 for n in proto.graph.node if n.op_type == op_type)
+
+        self.assertEqual(count_ops(model_proto, "Relu"), 1)
+        self.assertEqual(count_ops(model_proto, "Split"), 1)
+        self.assertEqual(count_ops(optimized, "Relu"), 2)
+        self.assertEqual(count_ops(optimized, "Split"), 1)
+
+
 if __name__ == "__main__":
     unittest.main()