Move Int4CPULayout to int4_cpu_layout.py (#1419)

* Move Int4CPULayout to int4_cpu_layout.py

* Apply automatic Ruff fixes

---------

Co-authored-by: Ruff Auto-fixes <[email protected]>

torchao/dtypes/uintx/__init__.py

@@ -1,6 +1,9 @@
 from .block_sparse_layout import (
     BlockSparseLayout,
 )
+from .int4_cpu_layout import (
+    Int4CPULayout,
+)
 from .marlin_qqq_tensor import (
     MarlinQQQLayout,
     MarlinQQQTensor,
@@ -13,7 +16,6 @@
     SemiSparseLayout,
 )
 from .tensor_core_tiled_layout import (
-    Int4CPULayout,
     TensorCoreTiledLayout,
 )
 from .uintx_layout import (

torchao/dtypes/uintx/int4_cpu_layout.py

@@ -0,0 +1,263 @@
+from dataclasses import dataclass
+from typing import Optional, Tuple
+
+import torch
+from torch.utils._python_dispatch import return_and_correct_aliasing
+
+from torchao.dtypes.affine_quantized_tensor import register_layout
+from torchao.dtypes.utils import AQTTensorImpl, Layout, is_device
+from torchao.utils import (
+    TORCH_VERSION_AT_LEAST_2_5,
+    TORCH_VERSION_AT_LEAST_2_6,
+    fill_defaults,
+)
+
+aten = torch.ops.aten
+
+
+@dataclass(frozen=True)
+class Int4CPULayout(Layout):
+    """Only for PyTorch version at least 2.6"""
+
+    pass
+
+
+@register_layout(Int4CPULayout)
+class Int4CPUAQTTensorImpl(AQTTensorImpl):
+    """
+    TensorImpl for int4 CPU layout for affine quantized tensor, this is for int4 only,
+    used by tinygemm kernels `_weight_int4pack_mm_for_cpu`
+    It stores the original tensor of dimension [n][k] (int32 dtype) as packed weight of 2-d tensor of
+    dimension: [n][k / 2] (uint8 dtype)
+    (unpacked Tensor shape is n * k)
+    Note: we also pack scale and zero point together here for tinygemm kernel
+    Note: technically Int4 CPU layout should be the layout for the underlying packed weight
+    (int Tensor) but since the scale and zero_point are also packed into the same tensor here which is not used
+    in plain layout, we just created a layout for AQT right now, this could be improved if we split out
+    int4 aqt into a separate tensor subclass
+    fields:
+      packed_weight (torch.Tensor): the 2-d packed tensor in a Int4 CPU layout
+      scale_and_zero (torch.Tensor): the combined scale Tensor used to map between floating point tensor to quantized tensor and zero_point Tensor
+    """
+
+    def __new__(
+        cls,
+        packed_weight: torch.Tensor,
+        scale_and_zero: torch.Tensor,
+        transposed: bool,
+        _layout: Layout,
+    ):
+        kwargs = {}
+        kwargs["device"] = packed_weight.device
+        kwargs["layout"] = (
+            kwargs.get("layout")
+            if kwargs.get("layout", False)
+            else packed_weight.layout
+        )
+        kwargs["dtype"] = packed_weight.dtype
+        kwargs["requires_grad"] = False
+        shape = packed_weight.shape
+        return torch.Tensor._make_wrapper_subclass(cls, shape, **kwargs)  # type: ignore[attr-defined]
+
+    def __init__(
+        self,
+        packed_weight: torch.Tensor,
+        scale_and_zero: torch.Tensor,
+        transposed: bool,
+        _layout: Layout,
+    ):
+        self.packed_weight = packed_weight
+        self.scale_and_zero = scale_and_zero
+        self.transposed = False
+        self._layout = _layout
+
+    def __tensor_flatten__(self):
+        return ["packed_weight", "scale_and_zero"], [self.transposed, self._layout]
+
+    @classmethod
+    def __tensor_unflatten__(
+        cls, tensor_data_dict, tensor_attributes, outer_size, outer_stride
+    ):
+        packed_weight, scale_and_zero = (
+            tensor_data_dict["packed_weight"],
+            tensor_data_dict["scale_and_zero"],
+        )
+        (
+            transposed,
+            _layout,
+        ) = tensor_attributes
+        return cls(packed_weight, scale_and_zero, transposed, _layout)
+
+    @classmethod
+    def from_plain(
+        cls,
+        int_data: torch.Tensor,
+        scale: torch.Tensor,
+        zero_point: Optional[torch.Tensor],
+        _layout: Layout,
+    ):
+        assert isinstance(_layout, Int4CPULayout)
+
+        if TORCH_VERSION_AT_LEAST_2_6:
+            assert (
+                int_data.dtype == torch.int32
+            ), "torch.ops.aten._convert_weight_to_int4pack_for_cpu expects `int32` dtype"
+            packed_weight = torch.ops.aten._convert_weight_to_int4pack_for_cpu(
+                int_data,
+                1,  # TODO:remove
+            )
+        elif TORCH_VERSION_AT_LEAST_2_5:
+            int_data = (int_data[::, ::2] << 4 | int_data[::, 1::2]).to(torch.uint8)
+            assert (
+                int_data.dtype == torch.uint8
+            ), "torch.ops.aten._convert_weight_to_int4pack in torch 2.5 expects `uint8` dtype"
+            packed_weight = torch.ops.aten._convert_weight_to_int4pack(
+                int_data, _layout.inner_k_tiles
+            )
+        else:
+            assert (
+                int_data.dtype == torch.int32
+            ), "torch.ops.aten._convert_weight_to_int4pack in torch 2.4 expects `int32` dtype"
+            packed_weight = torch.ops.aten._convert_weight_to_int4pack(
+                int_data, _layout.inner_k_tiles
+            )
+
+        scale = scale.reshape(int_data.shape[0], -1)
+        zero_point = zero_point.reshape(int_data.shape[0], -1)
+        from torchao.quantization.utils import pack_tinygemm_scales_and_zeros
+
+        scale_and_zero = pack_tinygemm_scales_and_zeros(scale, zero_point)
+        return cls(packed_weight, scale_and_zero, False, _layout)
+
+    def to(self, *args, **kwargs):
+        kwargs = self._get_to_kwargs(*args, **kwargs)
+        device = kwargs["device"]
+        if not is_device(torch.device(self.device).type, device):
+            raise ValueError(
+                f"Int4CPUAQTTensorImpl does not support conversion from {self.device} to {device}"
+            )
+        return self.__class__(
+            self.packed_weight.to(device),
+            self.scale_and_zero.to(device),
+            self.transposed,
+            self._layout,
+        )
+
+    def _apply_fn_to_data(self, fn):
+        return self.__class__(
+            fn(self.packed_weight),
+            fn(self.scale_and_zero),
+            self.transposed,
+            self._layout,
+        )
+
+    @classmethod
+    def __torch_dispatch__(cls, func, types, args, kwargs):
+        kwargs = {} if kwargs is None else kwargs
+
+        if func is aten.detach.default:
+            return return_and_correct_aliasing(
+                func, args, kwargs, args[0]._apply_fn_to_data(torch.detach)
+            )
+
+        if func is aten.clone.default:
+            return return_and_correct_aliasing(
+                func, args, kwargs, args[0]._apply_fn_to_data(torch.clone)
+            )
+
+        if func is aten.t.default:
+            """we don't need to repack the weight and just rely on external
+            shape being changed and record the status of transpose/no-transpose
+            """
+            transposed = Int4CPUAQTTensorImpl(
+                args[0].packed_weight,
+                args[0].scale_and_zero,
+                not args[0].transposed,
+                args[0]._layout,
+            )
+            return return_and_correct_aliasing(func, args, kwargs, transposed)
+
+        if func is aten.slice.Tensor:
+            self, dim, start, end, step = fill_defaults(args, 5, [0, None, None, 1])
+            if dim == 0:
+                int_data, scale, zero_point = self.get_plain()
+                int_data = aten.slice.Tensor(int_data, dim, start, end, step)
+                # this is to handle padding
+                int_data = self._layout.post_process(int_data)
+                sliced = self.from_plain(int_data, scale, zero_point, self._layout)
+                return return_and_correct_aliasing(func, args, kwargs, sliced)
+            elif dim == 1:
+                int_data, scale, zero_point = self.get_plain()
+                assert step == 1, "Only step == 1 is supported in slicing right now"
+                data_len = int_data.shape[dim]
+                scale_len = scale.shape[dim]
+                ratio = data_len / scale_len
+                start_scale = int(start / ratio)
+                end_scale = int(end / ratio)
+
+                int_data = aten.slice.Tensor(int_data, dim, start, end, step)
+                # this is to handle padding
+                int_data = self._layout.post_process(int_data)
+                scale = aten.slice.Tensor(scale, dim, start_scale, end_scale, step)
+                zero_point = aten.slice.Tensor(
+                    zero_point, dim, start_scale, end_scale, step
+                )
+                sliced = self.from_plain(int_data, scale, zero_point, self._layout)
+                return sliced
+            else:
+                raise NotImplementedError(
+                    f"Int4CPUAQTTensorImpl dispatch: attempting to run {func}, with dim={dim}, that is not supported"
+                )
+
+        raise NotImplementedError(
+            f"Int4CPUAQTTensorImpl dispatch: attempting to run {func}, this is not supported"
+        )
+
+    __torch_function__ = torch._C._disabled_torch_function_impl
+
+    def get_plain(self) -> Tuple[torch.Tensor, torch.Tensor, torch.Tensor]:
+        from torchao.quantization.quant_primitives import (
+            ZeroPointDomain,
+            quantize_affine,
+        )
+        from torchao.quantization.utils import unpack_tinygemm_scales_and_zeros
+
+        scale, zero = unpack_tinygemm_scales_and_zeros(self.scale_and_zero)
+
+        cur_shape = self.shape
+        assert len(cur_shape) == 2
+        original_shape = (cur_shape[0], cur_shape[1] * 2)
+        eye_shape = original_shape[1]
+        groupsize = int(original_shape[1] / scale.shape[-2])
+        block_size = (1, groupsize)
+        device = self.device
+        original_dtype = torch.bfloat16
+        target_dtype = torch.int32
+        quant_min = 0
+        quant_max = 15
+        zero_point_domain = ZeroPointDomain.FLOAT
+        assert len(block_size) == 2 and block_size[0] == 1
+        dequantized = torch.ops.aten._weight_int4pack_mm_for_cpu(
+            torch.eye(eye_shape, device=device, dtype=original_dtype),
+            self.packed_weight,
+            groupsize,
+            self.scale_and_zero,
+        )
+        dequantized = dequantized.t().contiguous()
+        # TODO: move this to `unpack_tinygemm_scales_and_zeros`?
+        scale = scale.reshape(scale.shape[:-1]).contiguous()
+        zero = zero.reshape(zero.shape[:-1]).contiguous()
+        int_data = quantize_affine(
+            dequantized,
+            block_size,
+            scale,
+            zero,
+            target_dtype,
+            quant_min,
+            quant_max,
+            zero_point_domain,
+        )
+        return int_data, scale, zero
+
+    def get_layout(self) -> Layout:
+        return self._layout