[Inductor] Lower fallback nodes annotated with "should_fallback" (#166339)

Summary:
This PR introduces an inductor-level fallback mechanism that gives users control over which operations or subgraphs Inductor should lower and which should fall back to preexisting kernels. This has similar motivation as #164776 in providing flexibility to selectively disable Inductor lowering for specific nodes.

The implementation simply adds a check for the `"should_fallback"` metadata annotation on FX graph nodes. If this is set to `True`, the lowerer falls back before attempting the normal lowering path. Note that since these are user-directed fallbacks dependent upon specific, customized conditions, use `add_to_fallback_set=False` to avoid permanent overwrites of inductor's lowering/fallback rules.

Simple example marking nodes for fallback based on custom predicates:

```
def should_fallback_predicate(node: torch.fx.Node, pred: Callable[torch.fx.Node, bool]):
    # Apply predicate and mark for fallback if needed
    if self.predicate(node):
         node.meta["should_fallback"] = True
```

Test Plan: added a CI test

Differential Revision: D85347587

Pull Request resolved: https://github.com/pytorch/pytorch/pull/166339
Approved by: https://github.com/blaine-rister, https://github.com/eellison

test/inductor/test_selective_lowering.py

@@ -0,0 +1,91 @@
+# Owner(s): ["module: inductor"]
+"""
+Test selective lowering control via node metadata annotations.
+"""
+
+from collections.abc import Callable
+
+import torch
+from torch._inductor.test_case import TestCase as InductorTestCase
+from torch.testing._internal.common_utils import instantiate_parametrized_tests
+from torch.testing._internal.inductor_utils import GPU_TYPE, HAS_GPU
+
+
+@instantiate_parametrized_tests
+class SelectiveLoweringTest(InductorTestCase):
+    """
+    Tests for user-controllable selective lowering using node.meta annotations.
+    """
+
+    device = GPU_TYPE
+
+    def _mark_nodes_for_fallback(
+        self, gm: torch.fx.GraphModule, predicate: Callable[[torch.fx.Node], bool]
+    ) -> torch.fx.GraphModule:
+        """
+        Helper method to mark nodes with should_fallback metadata based on a predicate.
+        """
+        for node in gm.graph.nodes:
+            if node.op == "call_function" and predicate(node):
+                node.meta["should_fallback"] = True
+        return gm
+
+    def test_basic_selective_lowering(self):
+        """
+        Test that nodes marked for fallback use fallback handlers instead of lowerings.
+        """
+
+        def foo(x, y):
+            a = x + y  # This will be marked for fallback
+            b = a * 2  # This will use normal lowering
+            return b
+
+        x = torch.randn(10, device=self.device)
+        y = torch.randn(10, device=self.device)
+
+        def custom_backend(gm: torch.fx.GraphModule, example_inputs):
+            # Mark all add operations for fallback
+            def should_fallback_add(node: torch.fx.Node) -> bool:
+                return node.target == torch.ops.aten.add.Tensor
+
+            self._mark_nodes_for_fallback(gm, should_fallback_add)
+
+            from torch._inductor.compile_fx import compile_fx
+
+            return compile_fx(gm, example_inputs)
+
+        compiled_fn = torch.compile(foo, backend=custom_backend)
+        result = compiled_fn(x, y)
+        expected = foo(x, y)
+
+        self.assertTrue(torch.allclose(result, expected))
+
+    def test_no_fallback_when_unmarked(self):
+        """
+        Test that operations without fallback annotation use normal lowering.
+        """
+
+        def foo(x, y):
+            return x + y
+
+        x = torch.randn(10, device=self.device)
+        y = torch.randn(10, device=self.device)
+
+        def custom_backend(gm: torch.fx.GraphModule, example_inputs):
+            # Don't mark anything - all operations should use normal lowering
+            from torch._inductor.compile_fx import compile_fx
+
+            return compile_fx(gm, example_inputs)
+
+        compiled_fn = torch.compile(foo, backend=custom_backend)
+        result = compiled_fn(x, y)
+        expected = foo(x, y)
+
+        self.assertTrue(torch.allclose(result, expected))
+
+
+if __name__ == "__main__":
+    from torch._inductor.test_case import run_tests
+
+    if HAS_GPU:
+        run_tests(needs="filelock")

torch/_inductor/graph.py

@@ -1322,6 +1322,11 @@ class GraphLowering(torch.fx.Interpreter):
                 else:
                     args, kwargs = layout_constraints(n, *args, **kwargs)
 
+            if "should_fallback" in n.meta:
+                out = fallback_handler(target, add_to_fallback_set=False)(
+                    *args, **kwargs
+                )
+            else:
                 out = lowerings[target](*args, **kwargs)  # type: ignore[index]
 
             if layout_constraints: