[ET-VK] Update RemoveLocalScalarDenseOpsTransform to tag scalar tensors as well

## Context

See the new docstrings added to `remove_local_scalar_dense_ops` for more details on what the pass is trying to achieve.

The goal is to mark tensors that are consumed as scalars via `tensor[0].item()` as "scalar tensors" that will be represented as a `SymInt` object in the vulkan delegate instead of a regular `Tensor` object.

This diff also adds an `__init__.py` file to the `_passes` folder to make it easier to include Vulkan passes from one place.

Differential Revision: [D64139867](https://our.internmc.facebook.com/intern/diff/D64139867/)

[ghstack-poisoned]

Author: SS-JIA

backends/vulkan/TARGETS

@@ -28,7 +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/backends/vulkan/_passes:vulkan_passes",
         "//executorch/exir:graph_module",
         "//executorch/exir/_serialize:_bindings",
         "//executorch/exir/_serialize:lib",

backends/vulkan/_passes/TARGETS

@@ -40,3 +40,16 @@ runtime.python_library(
         "//executorch/exir/dialects:lib",
     ],
 )
+
+runtime.python_library(
+    name = "vulkan_passes",
+    srcs = [
+        "__init__.py",
+    ],
+    visibility = [
+        "//executorch/backends/...",
+    ],
+    deps = [
+        ":remove_local_scalar_dense",
+    ]
+)

backends/vulkan/_passes/__init__.py

@@ -0,0 +1,7 @@
+from executorch.backends.vulkan._passes.remove_local_scalar_dense_ops import (
+    RemoveLocalScalarDenseOpsTransform,
+)
+
+__all__ = [
+    "RemoveLocalScalarDenseOpsTransform",
+]

backends/vulkan/_passes/remove_local_scalar_dense_ops.py

@@ -10,29 +10,95 @@
 from executorch.exir.dialects._ops import ops as exir_ops
 from executorch.exir.pass_base import ExportPass, PassResult
 
+from torch._subclasses.fake_tensor import FakeTensor
+
+
+def node_is_local_scalar_dense_chain(node: torch.fx.Node) -> bool:
+    """
+    Converting a tensor to a scalar via tensor[0].item() creates a index_select +
+    local_scalar_dense pattern in the graph. Check if a node is the start of this pattern.
+    """
+    if (
+        node.op == "call_function"
+        and node.target == exir_ops.edge.aten.select_copy.int
+        and len(node.users) == 1
+    ):
+        user = list(node.users.keys())[0]
+        return user.target == torch.ops.aten._local_scalar_dense.default
+
+    return False
+
+
+def tag_node_if_scalar_tensor(node: torch.fx.Node) -> None:
+    """
+    A scalar tensor in the Vulkan backend is a tensor that can be represented as a scalar
+    value instead of a Tensor object. The criteria for identifying a tensor as a scalar
+    tensor are as follows:
+
+    1. The tensor has only 1 element
+    2. One of the node's uses is converting it to a scalar via `tensor[0].item()`, which
+       creates a index_select + local_scalar_dense pattern in the graph
+
+    If any of these criteria are fulfilled, then tag the node for the tensor to mark it
+    so that it is added as a scalar value during serialization.
+    """
+    tensor_val = node.meta["val"]
+    if not isinstance(tensor_val, FakeTensor):
+        return
+
+    # Scalar tensors must have only one element
+    if tensor_val.numel() != 1:
+        return
+
+    for user in node.users:
+        if node_is_local_scalar_dense_chain(user):
+            node.meta["vkdg_is_scalar_tensor"] = True
+
+
+def remove_local_scalar_dense_chain(graph: torch.fx.Graph, node: torch.fx.Node) -> None:
+    """
+    Remove the index_select + local_scalar_dense pattern in the graph in favor of passing
+    the original scalar tensor directly.
+    """
+    replace_node = node.args[0]
+    assert isinstance(replace_node, torch.fx.Node)
+    # 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
+    ):
+        # pyre-ignore
+        if replace_node.args[0].meta["val"].numel() == 1:
+            replace_node = replace_node.args[0]
+            assert isinstance(replace_node, torch.fx.Node)
+            assert replace_node.meta.get("vkdg_is_scalar_tensor", True)
+
+    with graph.inserting_after(node):
+        node.replace_all_uses_with(replace_node)
+
 
 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.
+    The purpose of this pass is twofold:
+    1. Tag scalar tensors (see `tag_node_if_scalar_tensor()` for the criteria)
+    2. Remove the index_select + local_scalar_dense pattern in the graph in favor of
+       passing the original scalar tensor directly (see `remove_local_scalar_dense_chain()`)
+
+    This makes it easier to deal with scalar tensors in the Vulkan backend. In particular,
+    it allows serializing scalar tensors as SymInt objects instead of Tensor objects.
+    Because scalar tensors are often used to inform tensor shapes, their values need to
+    be easily accessed by the CPU during resizing logic, while also being able to reflect
+    updates to their value in any GPU shaders that reference them.
     """
     target_op = torch.ops.aten._local_scalar_dense.default
     for node in graph.nodes:
+        tag_node_if_scalar_tensor(node)
+
         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)
+            remove_local_scalar_dense_chain(graph, node)
 
     graph.eliminate_dead_code()
     return graph

backends/vulkan/vulkan_preprocess.py

@@ -17,9 +17,7 @@
 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._passes import RemoveLocalScalarDenseOpsTransform
 
 from executorch.backends.vulkan.serialization.vulkan_graph_builder import VkGraphBuilder
 from executorch.backends.vulkan.serialization.vulkan_graph_serialize import (