[ET-VK] Add pass to remove local_scalar_dense

Pull Request resolved: #5886

## Context

Scalar tensors (i.e. tensors with only 1 element) are often passed in to functions as scalars via

```
scalar_tensor[0].item()
```

This translates to the following chain in the graph

```
index_select = index_select(scalar_tensor, ...)
scalar = local_scalar_dense(index_select)
```

This diff introduces a pass to remove the `local_scalar_dense` "chain" in favor of passing in the input tensor directly.

Note that this replacement only occurs if the original tensor is a scalar tensor.

In the Vulkan backend, these scalar tensors will be represented as symbolic integers instead of actual tensors, which is why this replacement is valid. However, it may not a valid replacement for other backends.
ghstack-source-id: 246752220

Differential Revision: [D63913432](https://our.internmc.facebook.com/intern/diff/D63913432/)

Author: SS-JIA

backends/vulkan/TARGETS

@@ -28,6 +28,7 @@ runtime.python_library(
         "//executorch/backends/transforms:fuse_view_copy",
         "//executorch/backends/transforms:mean_to_sum_div",
         "//executorch/backends/transforms:remove_clone_ops",
+        "//executorch/backends/vulkan/passes:remove_local_scalar_dense",
         "//executorch/exir:graph_module",
         "//executorch/exir/_serialize:_bindings",
         "//executorch/exir/_serialize:lib",

backends/vulkan/partitioner/vulkan_partitioner.py

@@ -108,6 +108,31 @@ def is_linear_permute(self, node: torch.fx.Node) -> bool:
 
         return False
 
+    def is_in_local_scalar_dense_chain(self, node: torch.fx.Node) -> bool:
+        """
+        Scalar tensors are usually converted to scalar values in the graph via`
+        scalar_tensor[0].item()` in Python, which translates to a chain of
+        `local_scalar_dense(torch.select.int(scalar_tensor, 0, 0))` in the graph.
+        This function marks the entire chain as supported by the Vulkan delegate.
+
+        Later, within vulkan_preprocess there will be a graph transform which
+        replaces the chain with passing in the scalar tensor directly.
+        """
+        if node.target == exir_ops.edge.aten.select_copy.int:
+            if len(node.users) != 1:
+                return False
+            # pyre-ignore
+            if node.args[0].meta["val"].numel() != 1:
+                return False
+
+            user = list(node.users.keys())[0]
+            return user.target == torch.ops.aten._local_scalar_dense.default
+
+        if node.target == torch.ops.aten._local_scalar_dense.default:
+            return True
+
+        return False
+
     def is_node_supported(
         self, submodules: Mapping[str, torch.nn.Module], node: torch.fx.Node
     ) -> bool:
@@ -122,6 +147,9 @@ def _is_node_supported(
         if self.is_linear_permute(node):
             return True
 
+        if self.is_in_local_scalar_dense_chain(node):
+            return True
+
         if node.target not in VulkanSupportedOperators._ops:
             return False
 

backends/vulkan/passes/TARGETS

@@ -27,3 +27,16 @@ python_unittest(
         "//caffe2:torch",
     ],
 )
+
+runtime.python_library(
+    name = "remove_local_scalar_dense",
+    srcs = ["remove_local_scalar_dense_ops.py"],
+    visibility = [
+        "//executorch/backends/...",
+    ],
+    deps = [
+        "//caffe2:torch",
+        "//executorch/exir:pass_base",
+        "//executorch/exir/dialects:lib",
+    ],
+)

backends/vulkan/passes/remove_local_scalar_dense_ops.py

@@ -0,0 +1,44 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the BSD-style license found in the
+# LICENSE file in the root directory of this source tree.
+
+# pyre-strict
+
+import torch
+from executorch.exir.dialects._ops import ops as exir_ops
+from executorch.exir.pass_base import ExportPass, PassResult
+
+
+def remove_local_scalar_dense_ops(graph: torch.fx.Graph) -> torch.fx.Graph:
+    """
+    Remove local_scalar_dense op nodes and replace uses with parent node, or the
+    original scalar tensor.
+    """
+    target_op = torch.ops.aten._local_scalar_dense.default
+    for node in graph.nodes:
+        if node.op == "call_function" and node.target == target_op:
+            replace_node = node.args[0]
+            # If the argument to the local_scalar_dense op is a select op with only
+            # one user, and the argument to the select op is a tensor with only one
+            # element (i.e. a scalar tensor), then replace the entire pattern with the
+            # scalar tensor.
+            if (
+                replace_node.op == "call_function"
+                and replace_node.target == exir_ops.edge.aten.select_copy.int
+            ):
+                if replace_node.args[0].meta["val"].numel() == 1:
+                    replace_node = replace_node.args[0]
+
+            with graph.inserting_after(node):
+                node.replace_all_uses_with(replace_node)
+
+    graph.eliminate_dead_code()
+    return graph
+
+
+class RemoveLocalScalarDenseOpsTransform(ExportPass):
+    def call(self, graph_module: torch.fx.GraphModule) -> PassResult:
+        graph_module.graph = remove_local_scalar_dense_ops(graph_module.graph)
+        return PassResult(graph_module, True)

backends/vulkan/vulkan_preprocess.py

@@ -17,6 +17,10 @@
 from executorch.backends.transforms.mean_to_sum_div import MeanToSumDiv
 from executorch.backends.transforms.remove_clone_ops import RemoveCloneOpsTransform
 
+from executorch.backends.vulkan.passes.remove_local_scalar_dense_ops import (
+    RemoveLocalScalarDenseOpsTransform,
+)
+
 from executorch.backends.vulkan.serialization.vulkan_graph_builder import VkGraphBuilder
 from executorch.backends.vulkan.serialization.vulkan_graph_serialize import (
     serialize_vulkan_graph,
@@ -57,6 +61,7 @@ def preprocess(  # noqa: C901
             MeanToSumDiv(),
             SpecPropPass(),
             ConstraintBasedSymShapeEvalPass(),
+            RemoveLocalScalarDenseOpsTransform(),
             MemoryPlanningPass(),
         ]