FuseDequantLinearPass to convert dq -> linear into weight_int8pack_mm (#4708)

Summary:
Pull Request resolved: #4708

Replaces `dq(weight) -> linear(activation, dq)` with `weight_int8pack_mm`
Replaces `dq(weight) -> linear(activation, dq, bias)` with `weight_int8pack_mm -> add`

Reviewed By: copyrightly

Differential Revision: D60945766

Author: nathanaelsee

backends/transforms/TARGETS

@@ -73,6 +73,21 @@ runtime.python_library(
     ],
 )
 
+runtime.python_library(
+    name = "fuse_dequant_linear",
+    srcs = ["fuse_dequant_linear.py"],
+    visibility = [
+        "//executorch/backends/...",
+    ],
+    deps = [
+        ":utils",
+        "//caffe2:torch",
+        "//executorch/exir:pass_base",
+        "//executorch/exir:sym_util",
+        "//executorch/exir/dialects:lib",
+    ],
+)
+
 runtime.python_library(
     name = "fuse_view_copy",
     srcs = ["fuse_view_copy.py"],

backends/transforms/fuse_dequant_linear.py

@@ -0,0 +1,77 @@
+# 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
+
+
+class FuseDequantLinearPass(ExportPass):
+    """
+    Fuses weight dequantize_per_channel nodes with linear nodes into
+    weight_int8pack_mm nodes, for 8-bit weight-only quantization.
+
+    Replaces dq(weight) -> linear(activation, dq)       with weight_int8pack_mm
+    Replaces dq(weight) -> linear(activation, dq, bias) with weight_int8pack_mm -> add
+    """
+
+    def fuse_dequant_with_linear(
+        self,
+        graph_module: torch.fx.GraphModule,
+        dequant_node: torch.fx.Node,
+        linear_node: torch.fx.Node,
+    ) -> None:
+        activations = linear_node.args[0]
+        bias = None
+        if len(linear_node.args) > 2:
+            bias = linear_node.args[2]
+        quant_weight = dequant_node.args[0]
+        scale = dequant_node.args[1]
+
+        with graph_module.graph.inserting_before(linear_node):
+            weight_int8pack_mm_node = graph_module.graph.create_node(
+                "call_function",
+                exir_ops.edge.aten._weight_int8pack_mm.default,
+                (activations, quant_weight, scale),
+            )
+            if bias:
+                add_node = graph_module.graph.create_node(
+                    "call_function",
+                    exir_ops.edge.aten.add.Tensor,
+                    (weight_int8pack_mm_node, bias),
+                )
+                linear_node.replace_all_uses_with(add_node)
+            else:
+                linear_node.replace_all_uses_with(weight_int8pack_mm_node)
+            graph_module.graph.erase_node(linear_node)
+            graph_module.graph.erase_node(dequant_node)
+
+    def is_node_target(
+        self, node: torch.fx.Node, target: torch._ops.OperatorBase
+    ) -> bool:
+        return node.op == "call_function" and node.target == target
+
+    def call(self, graph_module: torch.fx.GraphModule) -> PassResult:
+        for node in graph_module.graph.nodes:
+            if self.is_node_target(node, exir_ops.edge.aten.linear.default):
+                weight_node = node.args[1]
+                if self.is_node_target(
+                    weight_node,
+                    exir_ops.edge.quantized_decomposed.dequantize_per_channel.default,
+                ):
+                    # only fuse if weight tensor is int8 packed
+                    quant_weight = weight_node.args[0]
+                    if quant_weight.meta["val"].dtype != torch.int8:
+                        continue
+                    self.fuse_dequant_with_linear(graph_module, weight_node, node)
+
+        graph_module.recompile()
+        graph_module = super().call(graph_module).graph_module
+
+        return PassResult(graph_module, True)