[aoti-et] Tag unused lifted constant placeholders

For lifted constants, if they are not being used, they won't be tagged
by `tag_constant_data()` API. We have to manually tag them in cuda
partitioner.

Author: larryliu0820

backends/aoti/aoti_partitioner.py

@@ -15,7 +15,7 @@
     PartitionResult,
 )
 from executorch.exir.backend.utils import tag_constant_data, tag_mutated_buffer
-from torch.export.exported_program import ExportedProgram
+from torch._export.utils import is_buffer, is_lifted_tensor_constant, is_param
 
 
 @experimental(
@@ -61,6 +61,18 @@ def partition(self, exported_program: ExportedProgram) -> PartitionResult:
         tag_constant_data(exported_program)
         tag_mutated_buffer(exported_program)
 
+        # Tag constant placeholders that have no users
+        # tag_constant_data only tags constants that have users with delegation_tag
+        # but we need to tag all constants for this partition
+        for node in exported_program.graph.nodes:
+            if node.op == "placeholder" and (
+                is_param(exported_program, node)
+                or is_buffer(exported_program, node)
+                or is_lifted_tensor_constant(exported_program, node)
+            ):
+                if "delegation_tag" not in node.meta:
+                    node.meta["delegation_tag"] = tag
+
         return PartitionResult(
             tagged_exported_program=exported_program, partition_tags=partition_tags
         )

backends/cuda/tests/test_cuda_partitioner.py

@@ -139,3 +139,86 @@ def forward(self, x: torch.Tensor) -> torch.Tensor:
             fully_partitioned,
             "Graph should be fully partitioned with all operators having the same tag",
         )
+
+    def test_unused_constant_tagging(self):
+        """
+        Test that constant nodes without users are properly tagged with delegation_tag.
+
+        When a graph contains constants (parameters, buffers, or lifted tensor constants)
+        that are not used by any operations, the CUDA partitioner should still tag them
+        with the delegation_tag. This ensures all constant data is properly handled during
+        delegation, even if they have no users in the graph.
+        """
+
+        class ModuleWithUnusedConst(torch.nn.Module):
+            def __init__(self):
+                super().__init__()
+                # Register a buffer that won't be used in forward
+                self.register_buffer("unused_buffer", torch.randn(10, 10))
+                # Also register a used parameter
+                self.weight = torch.nn.Parameter(torch.randn(5, 5))
+
+            def forward(self, x: torch.Tensor) -> torch.Tensor:
+                # Only use the weight parameter, not the unused_buffer
+                return x + self.weight
+
+        module = ModuleWithUnusedConst()
+        inputs = (torch.randn(5, 5),)
+
+        # Get partition result
+        partition_result = self._get_partition_result(module, inputs)
+
+        # Find all placeholder nodes (these represent constants, parameters, buffers, and inputs)
+        constant_placeholders = []
+        input_placeholders = []
+
+        for node in partition_result.tagged_exported_program.graph.nodes:
+            if node.op == "placeholder":
+                # Check if this is a constant (param, buffer, or lifted tensor constant)
+                from torch._export.utils import (
+                    is_buffer,
+                    is_lifted_tensor_constant,
+                    is_param,
+                )
+
+                is_constant = (
+                    is_param(partition_result.tagged_exported_program, node)
+                    or is_buffer(partition_result.tagged_exported_program, node)
+                    or is_lifted_tensor_constant(
+                        partition_result.tagged_exported_program, node
+                    )
+                )
+
+                if is_constant:
+                    constant_placeholders.append(node)
+                else:
+                    input_placeholders.append(node)
+
+        # Verify we have constant placeholders
+        self.assertGreater(
+            len(constant_placeholders),
+            0,
+            "Expected to find constant placeholders in the graph",
+        )
+
+        # Check that all constant placeholders are tagged, including unused ones
+        untagged_constants = []
+        for node in constant_placeholders:
+            if "delegation_tag" not in node.meta:
+                untagged_constants.append(node.name)
+
+        self.assertEqual(
+            len(untagged_constants),
+            0,
+            f"All constant placeholders should be tagged. Found untagged constants: {untagged_constants}",
+        )
+
+        # Verify all tagged constants have the expected tag
+        expected_tag = "tag0"
+        for node in constant_placeholders:
+            actual_tag = node.meta.get("delegation_tag")
+            self.assertEqual(
+                actual_tag,
+                expected_tag,
+                f"Constant placeholder {node.name} has tag '{actual_tag}' but expected '{expected_tag}'",
+            )