Qualcomm AI Engine Direct - raise ValueError for 2-bit per-tensor encoding

Summary:
raise ValueError for 2-bit per-tensor encoding in node_visitor and validators

Test plan:
python backends/qualcomm/tests/test_qnn_delegate.py TestQNNQuantizedOperator.test_qnn_backend_16a2w_conv2d -b build-android -H ${HOST} -s ${SN} -m SM8850
python backends/qualcomm/tests/test_qnn_delegate.py TestQNNQuantizedOperator.test_qnn_backend_16a2w_linear -b build-android -H ${HOST} -s ${SN} -m SM8850

Author: jethroqti

backends/qualcomm/builders/node_visitor.py

@@ -293,16 +293,15 @@ def make_qnn_per_tensor_config(self, quant_attrs: Dict):
         }
         # check Qnn_ScaleOffset_t in QNN/include/QnnTypes.h
         quant_config[QCOM_OFFSET] = -quant_attrs[QCOM_ZERO_POINT]
-        range_ = quant_config[QCOM_QUANT_MAX] - quant_config[QCOM_QUANT_MIN]
-        assert range_ > 3, (
-            f"2-bit quantization (range={range_}) does not support per-tensor encoding. "
-            "Use per-channel quantization instead."
-        )
-        # special case for 4 bits
-        if (
-            quant_config[QCOM_DTYPE] == torch.int8
-            and quant_config[QCOM_QUANT_MAX] - quant_config[QCOM_QUANT_MIN] <= 15
-        ):
+        # special case for 4-bit / 2-bit integer weights.
+        quant_range = quant_config[QCOM_QUANT_MAX] - quant_config[QCOM_QUANT_MIN]
+        if quant_config[QCOM_DTYPE] == torch.int8 and quant_range <= 15:
+            if quant_range <= 3:
+                raise ValueError(
+                    f"2-bit quantization (range={quant_range}) "
+                    "does not support per-tensor encoding. Use per-channel quantization instead."
+                )
+            # special case for 4 bits
             quant_config[QCOM_BITWIDTH] = 4
             return (
                 PyQnnManager.Qnn_QuantizationEncoding_t.QNN_QUANTIZATION_ENCODING_BW_SCALE_OFFSET,

backends/qualcomm/quantizer/validators.py

@@ -283,12 +283,21 @@ def _qspec_port_encoding_type(node: Node, qspec: QuantizationSpecBase):
     qscheme = qspec.qscheme
 
     if qscheme in [torch.per_tensor_symmetric, torch.per_tensor_affine]:
-        range_ = qspec.quant_max - qspec.quant_min
-        assert range_ > 3, (
-            f"2-bit quantization (range={range_}) does not support per-tensor encoding. "
-            "Use per-channel quantization instead."
-        )
-        if qspec.dtype == torch.int8 and range_ <= 15:
+        # quant_max/quant_min are None for non-integer activations (e.g. uint16 in
+        # 16a2w) whose range is not expressed as a fixed integer bound; skip the
+        # 4-bit BW_SCALE_OFFSET special-casing for those tensors.
+        if (
+            qspec.dtype == torch.int8
+            and qspec.quant_max is not None
+            and qspec.quant_min is not None
+            and (quant_range := qspec.quant_max - qspec.quant_min) <= 15
+        ):
+            if quant_range <= 3:
+                raise ValueError(
+                    f"2-bit quantization (range={quant_range}) "
+                    "does not support per-tensor encoding. "
+                    "Use per-channel quantization instead."
+                )
             encoding_type = (
                 PyQnnManager.Qnn_QuantizationEncoding_t.QNN_QUANTIZATION_ENCODING_BW_SCALE_OFFSET
             )
@@ -303,10 +312,6 @@ def _qspec_port_encoding_type(node: Node, qspec: QuantizationSpecBase):
             encoding_type = (
                 PyQnnManager.Qnn_QuantizationEncoding_t.QNN_QUANTIZATION_ENCODING_BLOCKWISE_EXPANSION
             )
-        elif qspec.dtype == torch.int8 and qspec.quant_max - qspec.quant_min <= 3:
-            encoding_type = (
-                PyQnnManager.Qnn_QuantizationEncoding_t.QNN_QUANTIZATION_ENCODING_BW_AXIS_SCALE_OFFSET
-            )
         elif qspec.dtype == torch.int8 and qspec.quant_max - qspec.quant_min <= 15:
             encoding_type = (
                 PyQnnManager.Qnn_QuantizationEncoding_t.QNN_QUANTIZATION_ENCODING_BW_AXIS_SCALE_OFFSET