Qualcomm AI Engine Direct - Add fp16a8w quantization config (pytorch#19537)

### Summary:
- Add fp16a8w quantization config
- Note that fp16a8w is only supported with Conv2d (kernel size = 1) and
Linear by QNN HTP
- Add a pass `insert_cast_for_fp_act_quantized_weight.py` to cast fp32
-> fp16 due to constraint in QNN HTP
- Add a test case to run conv2d and linear with fp16a8w

### Test plan
```
python3 backends/qualcomm/tests/test_qnn_delegate.py TestQNNFloatingPointOperator.test_qnn_backend_fp16a8w_simple_model  -b build-android -H ${HOST} -s ${DEVICE}  -m SM8750 -r /path/to/executorch -a /path/to/artifacts
```

Author: shewu-quic

backends/qualcomm/_passes/__init__.py

@@ -44,6 +44,7 @@
 from .fuse_consecutive_cast import FuseConsecutiveCast
 from .fuse_consecutive_transpose import FuseConsecutiveTranspose
 from .i64_to_i32 import I64toI32
+from .insert_cast_for_fp_act_quantized_weight import InsertCastForFpActQuantizedWeight
 from .insert_io_qdq import InsertIOQDQ
 from .insert_requantize import InsertRequantize
 from .insert_reshape_for_reduce_ops import InsertReshapeForReduceOps
@@ -102,6 +103,7 @@
     FuseConsecutiveCast,
     FuseConsecutiveTranspose,
     I64toI32,
+    InsertCastForFpActQuantizedWeight,
     InsertIOQDQ,
     InsertReshapeForReduceOps,
     InsertRequantize,

backends/qualcomm/_passes/insert_cast_for_fp_act_quantized_weight.py

@@ -0,0 +1,141 @@
+# Copyright (c) Qualcomm Innovation Center, Inc.
+# 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.
+
+import torch
+from executorch.backends.qualcomm.builders.node_visitor import dq_ops
+from executorch.backends.qualcomm.builders.utils import is_parameter
+from executorch.backends.qualcomm.utils.constants import QCOM_QUANT_ATTRS
+from executorch.exir.dialects._ops import ops as exir_ops
+from executorch.exir.pass_base import ExportPass, PassResult
+from executorch.exir.passes import dead_code_elimination_pass
+
+from .utils import copy_meta
+
+TARGET_OPS = {
+    exir_ops.edge.aten.convolution.default,
+    exir_ops.edge.aten.linear.default,
+}
+
+
+class InsertCastForFpActQuantizedWeight(ExportPass):
+    """
+    Insert fp32↔fp16 casts around conv/linear nodes that have a quantized
+    weight but a floating-point activation.
+
+    Background — QNN vs PyTorch dtype contract:
+      In PyTorch, a conv/linear with fp32 activation and int8 weight (e.g.
+      produced by fp16a8w quantization) is valid: the weight is stored as int8
+      but dequantized to fp32 before the multiply-accumulate.  QNN HTP, however,
+      requires that when the weight is quantized (int8/int4) the activation must
+      also be fp16, not fp32.  Passing an fp32 activation to such an op causes a
+      QNN compilation error.
+
+    Fix:
+      Wrap the offending node with an fp32→fp16 cast on the input activation and
+      an fp16→fp32 cast on the output, so the node itself operates in fp16 while
+      the surrounding graph continues to see fp32 tensors.
+
+      Before:  [fp32 act] → conv/linear(w=int8) → [fp32 out]
+      After:   [fp32 act] → cast(fp16) → conv/linear(w=int8) → cast(fp32) → [fp32 out]
+
+    Pattern matched:
+      - Node target is in TARGET_OPS (convolution, linear)
+      - Node has no QCOM_QUANT_ATTRS (activation is not quantized, i.e. fp32)
+      - Weight arg (args[1]) is a parameter with QCOM_QUANT_ATTRS,
+        optionally wrapped in a dequantize op
+      - Input activation dtype is fp32
+
+    The bias meta["val"] is also updated to fp16 to stay consistent with the
+    fp16 compute domain of the node.
+    """
+
+    def __init__(self, edge_program: torch.export.ExportedProgram):
+        super().__init__()
+        self.edge_program = edge_program
+
+    def _get_weight_param_node(self, weight: torch.fx.Node):
+        """Return the underlying parameter node for a weight, unwrapping a DQ op if present."""
+        if is_parameter(weight, self.edge_program):
+            return weight
+        if weight.target in dq_ops:
+            param_node = weight.args[0]
+            if isinstance(param_node, torch.fx.Node) and is_parameter(
+                param_node, self.edge_program
+            ):
+                return param_node
+        return None
+
+    def _has_quantized_weight(self, node: torch.fx.Node) -> bool:
+        if node.target not in TARGET_OPS or len(node.args) < 2:
+            return False
+        weight = node.args[1]
+        if not isinstance(weight, torch.fx.Node):
+            return False
+        param_node = self._get_weight_param_node(weight)
+        return param_node is not None and bool(param_node.meta.get(QCOM_QUANT_ATTRS))
+
+    def _insert_fp32_fp16_casts(
+        self, graph_module: torch.fx.GraphModule, node: torch.fx.Node
+    ):
+        """Wrap node with cast(fp32→fp16) on input and cast(fp16→fp32) on output."""
+        input_act = node.args[0]
+
+        with graph_module.graph.inserting_before(node):
+            cast_in = graph_module.graph.create_node(
+                "call_function",
+                exir_ops.edge.aten._to_copy.default,
+                (input_act,),
+                {"dtype": torch.float16},
+            )
+            cast_in.meta = copy_meta(
+                node.meta,
+                lambda m: {**m, "val": input_act.meta["val"].to(torch.float16)},
+            )
+        node.replace_input_with(input_act, cast_in)
+
+        # Update bias meta["val"] to fp16 if present.
+        if len(node.args) > 2 and node.args[2] is not None:
+            bias_node = node.args[2]
+            if isinstance(bias_node, torch.fx.Node) and "val" in bias_node.meta:
+                if bias_node.meta["val"].dtype == torch.float32:
+                    bias_node.meta["val"] = bias_node.meta["val"].to(torch.float16)
+
+        users = list(node.users.keys())
+        orig_output_val = node.meta["val"]
+        node.meta["val"] = orig_output_val.to(torch.float16)
+
+        with graph_module.graph.inserting_after(node):
+            cast_out = graph_module.graph.create_node(
+                "call_function",
+                exir_ops.edge.aten._to_copy.default,
+                (node,),
+                {"dtype": torch.float32},
+            )
+            cast_out.meta = copy_meta(
+                node.meta,
+                lambda m: {**m, "val": orig_output_val.to(torch.float32)},
+            )
+
+        for user in users:
+            user.replace_input_with(node, cast_out)
+
+    def call(self, graph_module: torch.fx.GraphModule):
+        for node in list(graph_module.graph.nodes):
+            if node.meta.get(QCOM_QUANT_ATTRS):
+                continue
+            if not self._has_quantized_weight(node):
+                continue
+            input_act = node.args[0]
+            if not isinstance(input_act, torch.fx.Node):
+                continue
+            input_val = input_act.meta.get("val")
+            if input_val is not None and input_val.dtype == torch.float32:
+                self._insert_fp32_fp16_casts(graph_module, node)
+
+        graph_module.graph.eliminate_dead_code()
+        graph_module.recompile()
+        dead_code_elimination_pass(graph_module)
+        return PassResult(graph_module, True)

backends/qualcomm/_passes/qnn_pass_manager.py

@@ -49,6 +49,7 @@
     FuseConsecutiveCast,
     FuseConsecutiveTranspose,
     I64toI32,
+    InsertCastForFpActQuantizedWeight,
     InsertIOQDQ,
     InsertRequantize,
     InsertReshapeForReduceOps,
@@ -120,6 +121,7 @@ def get_capture_program_passes():
         (FixedLinearKeepDim, True),
         (FoldQDQ, True),
         (I64toI32, True),
+        (InsertCastForFpActQuantizedWeight, True),
         (LayoutTransform, True),
         (RecomposePadMaxPool2d, True),
         (RecomposePixelUnshuffle, True),

backends/qualcomm/_passes/utils.py

@@ -80,6 +80,7 @@ def get_passes_dependency_for_capture_program():
         FixedLinearKeepDim,
         FoldQDQ,
         I64toI32,
+        InsertCastForFpActQuantizedWeight,
         LayoutTransform,
         RecomposePadMaxPool2d,
         RecomposePixelUnshuffle,
@@ -114,6 +115,7 @@ def get_passes_dependency_for_capture_program():
         FixedLinearKeepDim: [FoldQDQ],
         FoldQDQ: [AnnotateQuantAttrs, AnnotateStack, AnnotateUnbind],
         I64toI32: [RemoveRedundancy],
+        InsertCastForFpActQuantizedWeight: [FoldQDQ, LayoutTransform],
         LayoutTransform: [
             AnnotateQuantAttrs,
             ExpandBroadcastTensorShape,