Clean up linear_int8_dynamic_activation_intx_weight_subclass

Summary:
Cleans up layout and quantization API:

```
int8_dynamic_activation_intx_weight(
    group_size: int = 128,
    bit_width: int = 4,
    has_weight_zeros: bool = False,
    weight_mapping_type=MappingType.ASYMMETRIC,
    act_mapping_type=MappingType.ASYMMETRIC,
    layout=PackedLinearInt8DynamicActivationIntxWeightLayout(),
)
```

int8_dynamic_activation_intx_weight is now very similar to int8_dynamic_activation_int4_weight.  By passing bit_width=4, has_weight_zeros=false, and  layout=PlainLayout(), it should be numerically identical (but slower).

The fallback option is removed and instead relies on using PlainLayout().

Reviewed By: jerryzh168

Differential Revision: D67821939

torchao/_models/llama/generate.py

@@ -543,32 +543,18 @@ def ffn_or_attn_only(mod, fqn):
             from torchao.experimental.quant_api import (
                 int8_dynamic_activation_intx_weight,
             )
-
-            assert (
-                precision == torch.float32
-            ), "int8_dynamic_activation_intx_weight requires fp32 precision"
-
-            try:
-                torch.ops.torchao._pack_8bit_act_4bit_weight
-            except:
-                print(
-                    "Unable to load experimental torchao kernels.  Performance will be slow."
-                )
-                print(
-                    "To install the kernels, run `USE_CPP=1 pip install .` from ao on a machine with an ARM CPU"
-                )
+            assert precision == torch.float32, "int8_dynamic_activation_intx_weight requires using precision=torch.float32"
 
             # Quantize model
             _quant_args = quantization.split("-")
-            nbit = int(_quant_args[1])
-            assert nbit >= 1 and nbit <= 8, "nbits must be 1 to 8"
+            bit_width = int(_quant_args[1])
             group_size = int(_quant_args[2])
             has_weight_zeros = bool(_quant_args[3])
             quantize_(
                 model,
                 int8_dynamic_activation_intx_weight(
+                    bit_width=bit_width,
                     group_size=group_size,
-                    nbit=nbit,
                     has_weight_zeros=has_weight_zeros,
                 ),
             )

torchao/dtypes/uintx/plain_layout.py

@@ -38,7 +38,7 @@ def __new__(
         cls,
         int_data: torch.Tensor,
         scale: torch.Tensor,
-        zero_point: torch.Tensor,
+        zero_point: Optional[torch.Tensor],
         _layout: Layout,
     ):
         kwargs = {}
@@ -55,7 +55,7 @@ def __init__(
         self,
         int_data: torch.Tensor,
         scale: torch.Tensor,
-        zero_point: torch.Tensor,
+        zero_point: Optional[torch.Tensor],
         _layout: Layout,
     ):
         self.int_data = int_data
@@ -64,6 +64,8 @@ def __init__(
         self._layout = _layout
 
     def __tensor_flatten__(self):
+        if self.zero_point is None:
+            return ["int_data", "scale"], [self._layout]
         return ["int_data", "scale", "zero_point"], [self._layout]
 
     @classmethod
@@ -73,7 +75,7 @@ def __tensor_unflatten__(
         int_data, scale, zero_point = (
             tensor_data_dict["int_data"],
             tensor_data_dict["scale"],
-            tensor_data_dict["zero_point"],
+            tensor_data_dict.get("zero_point", None),
         )
         (_layout,) = tensor_attributes
         return cls(int_data, scale, zero_point, _layout)
@@ -83,15 +85,17 @@ def to(self, *args, **kwargs):
         return self.__class__(
             self.int_data.to(kwargs["device"]),
             self.scale.to(kwargs["device"]),
-            self.zero_point.to(kwargs["device"]),
+            self.zero_point.to(kwargs["device"])
+            if self.zero_point is not None
+            else None,
             self._layout,
         )
 
     def _apply_fn_to_data(self, fn):
         return self.__class__(
             fn(self.int_data),
             fn(self.scale),
-            fn(self.zero_point),
+            fn(self.zero_point) if self.zero_point is not None else None,
             self._layout,
         )
 
@@ -134,7 +138,7 @@ def __torch_dispatch__(cls, func, types, args, kwargs):
                 return PlainAQTTensorImpl(
                     aten.slice.Tensor(self.int_data, dim, start, end, step),
                     self.scale.view(-1),
-                    self.zero_point.view(-1),
+                    self.zero_point.view(-1) if self.zero_point is not None else None,
                     self._layout,
                 )
             else:
@@ -148,7 +152,7 @@ def __torch_dispatch__(cls, func, types, args, kwargs):
 
     __torch_function__ = torch._C._disabled_torch_function_impl
 
-    def get_plain(self) -> Tuple[torch.Tensor, torch.Tensor, torch.Tensor]:
+    def get_plain(self) -> Tuple[torch.Tensor, torch.Tensor, Optional[torch.Tensor]]:
         return self.int_data, self.scale, self.zero_point
 
     def get_layout(self) -> Layout:

torchao/dtypes/utils.py

@@ -1,5 +1,5 @@
 from dataclasses import dataclass
-from typing import Tuple, Union
+from typing import Optional, Tuple, Union
 
 import torch
 
@@ -87,7 +87,7 @@ class AQTTensorImpl(TorchAOBaseTensor):
     the underlying implementation of a AQT based on layout
     """
 
-    def get_plain(self) -> Tuple[torch.Tensor, torch.Tensor, torch.Tensor]:
+    def get_plain(self) -> Tuple[torch.Tensor, torch.Tensor, Optional[torch.Tensor]]:
         """Get the plain (unpacked) Tensor for the tensor impl
 
         Returns data, scale and zero_point
@@ -103,7 +103,7 @@ def from_plain(
         cls,
         data: torch.Tensor,
         scale: torch.Tensor,
-        zero_point: torch.Tensor,
+        zero_point: Optional[torch.Tensor],
         _layout: Layout,
     ):
         """Construct a TensorImpl from data, scale, zero_point and the _layout"""