Add generic fake quantized linear for QAT

Summary: This commit adds a generic fake quantized linear module
to replace the uses of the existing more specific QAT linears.
For example, `Int8DynActInt4WeightQATLinear` can be expressed
as follows:

```
from torchao.quantization.prototype.qat.api import FakeQuantizeConfig
from torchao.quantization.prototype.qat.linear import FakeQuantizedLinear

activation_config = FakeQuantizeConfig(torch.int8, "per_token", is_symmetric=False)
weight_config = FakeQuantizeConfig(torch.int4, group_size=8)
fq_linear = FakeQuantizedLinear(16, 32, False, activation_config, weight_config)
```

The main motivation is to provide a more flexible way to perform
QAT on models with linear layers. Previously, we would have to
create a new linear class every time we wish to experiment with
different fake quantization settings, e.g. different group size
or different bit width. Now we can express this easily using a
single linear module.

Test Plan:
python test/quantization/test_qat.py -k test_fake_quantize_config_granularity
python test/quantization/test_qat.py -k test_fake_quantize_config_granularity_error_cases
python test/quantization/test_qat.py -k test_fake_quantize_config_mapping_type
python test/quantization/test_qat.py -k test_fake_quantized_linear_8da4w
python test/quantization/test_qat.py -k test_fake_quantized_linear_4w

ghstack-source-id: 3a373be2b345776dc5b10b1677d3719d1071ff6a
Pull Request resolved: #1020

torchao/quantization/README.md

@@ -136,8 +136,7 @@ change_linear_weights_to_int8_dqtensors(model)
 
 ```python
 # for torch 2.4+
-from torchao.quantization import quantize_, float8_dynamic_activation_float8_weight
-from torchao.quantization.quant_api import PerTensor
+from torchao.quantization import quantize_, float8_dynamic_activation_float8_weight, PerTensor
 quantize_(model, float8_dynamic_activation_float8_weight(granularity=PerTensor()))
 ```
 
@@ -321,7 +320,7 @@ This API works today but has not been extensively tested and benchmarked yet. Ha
 
 ```python
 # for torch 2.5+
-from torchao.quantization.quant_api import quantize_, PerRow, float8_dynamic_activation_float8_weight
+from torchao.quantization import quantize_, PerRow, float8_dynamic_activation_float8_weight
 quantize_(model, float8_dynamic_activation_float8_weight(granularity=PerRow()))
 ```
 

torchao/quantization/__init__.py

@@ -12,11 +12,12 @@
 from .weight_only import *  # noqa: F403
 from .unified import *
 from .autoquant import *
-from .linear_activation_quantized_tensor import (  # noqat: F403
+from .granularity import *
+from .linear_activation_quantized_tensor import (
     LinearActivationQuantizedTensor,
     to_linear_activation_quantized,
 )
-from .linear_activation_scale import (  # noqat: F403
+from .linear_activation_scale import (
     to_weight_tensor_with_linear_activation_scale_metadata,
 )
 

torchao/quantization/granularity.py

@@ -22,7 +22,7 @@ class PerTensor(Granularity):
     """
     Represents per-tensor granularity in quantization.
 
-    This granularity type calcualtes the quantization parameters
+    This granularity type calculates the quantization parameters
     based off the entire tensor.
     """
     pass
@@ -32,26 +32,24 @@ class PerAxis(Granularity):
     """
     Represents per-axis granularity in quantization.
 
-    This granularity type calcualtes different quantization parameters
+    This granularity type calculates different quantization parameters
     along a specified axis of the tensor.
 
     For example if the input tensor is shape [8, 16] and axis=0, then
     the quantization parameters are calculated for each row of the tensor.
     Giving a total of 8 quantization parameters.
 
-
     Attributes:
         axis (int): The axis along which reduction is performed.
     """
     axis: int
 
 @dataclass(frozen=True)
-
 class PerGroup(Granularity):
     """
     Represents per-channel group granularity in quantization.
 
-    This granularity type calcualtes different quantization parameters
+    This granularity type calculates different quantization parameters
     for each group of <group_size> elements.
 
     For example if the input tensor is shape [8, 16], and the group size is 4, then
@@ -74,3 +72,19 @@ class PerRow(Granularity):
     is quantized with a block_size of (1, weight.shape[1]).
     """
     pass
+
+class PerToken(Granularity):
+    """
+    Represents per-token granularity in quantization.
+
+    This granularity type calculates a different set of quantization parameters
+    for each token, which is represented as the last dimension of the tensor.
+
+    For example, if the input tensor has shape [2, 3, 4], then there are 6 tokens
+    with 4 elements each, and we will calculate 6 sets of quantization parameters,
+    one for each token.
+
+    If the input tensor has only two dimensions, e.g. [8, 16], then this is
+    equivalent to `PerAxis(axis=0)`, which yields 8 sets of quantization parameters.
+    """
+    pass

torchao/quantization/prototype/qat/api.py

@@ -4,11 +4,224 @@
 # This source code is licensed under the license found in the
 # LICENSE file in the root directory of this source tree.
 
-from typing import Any, List
+from dataclasses import dataclass
+from enum import Enum
+from typing import Any, List, Optional, Union
 
 import torch
 
+from torchao.quantization.granularity import (
+    Granularity,
+    PerAxis,
+    PerGroup,
+    PerToken,
+)
 from torchao.quantization.unified import TwoStepQuantizer
+from torchao.quantization.quant_primitives import (
+    _SUB_BYTE_INT_BOUNDS,
+    _SUB_BYTE_UINT_BOUNDS,
+    MappingType,
+    TorchAODType,
+    ZeroPointDomain,
+)
+
+
+@dataclass
+class FakeQuantizeConfig:
+    """
+    Config for how to fake quantize weights or activations.
+
+    args:
+        dtype: dtype to simulate during fake quantization, e.g. torch.int8.
+            For PyTorch versions older than 2.6, you may use `TorchAODType` to represent
+            torch.int1 to torch.int7 instead, e.g. TorchAODType.INT4.
+        granularity: granularity of scales and zero points, e.g. PerGroup(32).
+            We also support the following strings:
+               1) 'per_token': equivalent to PerToken()
+               2) 'per_channel': equivalent to PerAxis(0)
+               3) 'per_group': equivalent to PerGroup(group_size), must be combined
+                   with separate `group_size` kwarg, Alternatively, just set the
+                   `group_size` kwarg and leave this field empty.
+        mapping_type: whether to use symmetric (default) or asymmetric quantization
+            Alternatively, set `is_symmetric` (bool) and leave this field empty.
+        scale_precision: scale dtype (default torch.fp32)
+        zero_point_precision: zero point dtype (default torch.int32)
+        zero_point_domain: whether zero point is in integer (default) or float domain
+        is_dynamic: whether to use dynamic (defualt) or static scale and zero points
+        range_learning: whether to learn scale and zero points during training (coming soon)
+
+    kwargs (optional):
+        group_size: size of each group in per group fake quantization,
+            can be set instead of `granularity`
+        is_symmetric: whether to use symmetric or asymmetric quantization,
+            can be set instead of `mapping_type`
+
+    Example usage::
+
+        # Per token asymmetric quantization
+        FakeQuantizeConfig(torch.int8, "per_token", is_symmetric=False)
+        FakeQuantizeConfig(torch.int8, PerToken(), MappingType.ASYMMETRIC)
+
+        # Per channel symmetric quantization
+        FakeQuantizeConfig(torch.int4, "per_channel")
+        FakeQuantizeConfig(torch.int4, "per_channel", is_symmetric=True)
+        FakeQuantizeConfig(torch.int4, PerAxis(0), MappingType.SYMMETRIC)
+
+        # Per group symmetric quantization
+        FakeQuantizeConfig(torch.int4, group_size=32)
+        FakeQuantizeConfig(torch.int4, group_size=32, is_symmetric=True)
+        FakeQuantizeConfig(torch.int4, "per_group", group_size=32, is_symmetric=True)
+        FakeQuantizeConfig(torch.int4, PerGroup(32), MappingType.SYMMETRIC)
+    """
+    dtype: Union[torch.dtype, TorchAODType]
+    granularity: Granularity
+    mapping_type: MappingType
+    scale_precision: torch.dtype
+    zero_point_precision: torch.dtype
+    zero_point_domain: ZeroPointDomain
+    is_dynamic: bool = True
+    range_learning: bool = False
+
+    def __init__(
+        self,
+        dtype: Union[torch.dtype, TorchAODType],
+        granularity: Union[Granularity, str, None] = None,
+        mapping_type: Optional[MappingType] = None,
+        scale_precision: torch.dtype = torch.float32,
+        zero_point_precision: torch.dtype = torch.int32,
+        zero_point_domain: ZeroPointDomain = ZeroPointDomain.INT,
+        is_dynamic: bool = True,
+        range_learning: bool = False,
+        *,
+        group_size: Optional[int] = None,
+        is_symmetric: Optional[bool] = None,
+    ):
+        self.dtype = dtype
+        self.granularity = self._get_granularity(granularity, group_size)
+        self.mapping_type = self._get_mapping_type(mapping_type, is_symmetric)
+        self.scale_precision = scale_precision
+        self.zero_point_precision = zero_point_precision
+        self.zero_point_domain = zero_point_domain
+        self.is_dynamic = is_dynamic
+        self.range_learning = range_learning
+
+        # Validate dtype
+        all_dtypes = [torch.int8, torch.uint8]
+        all_dtypes.extend(list(_SUB_BYTE_INT_BOUNDS.keys()))
+        all_dtypes.extend(list(_SUB_BYTE_UINT_BOUNDS.keys()))
+        if dtype not in all_dtypes:
+            raise ValueError("Unsupported dtype '%s', choose from %s" % (dtype, all_dtypes))
+
+    def _get_granularity(
+        self,
+        granularity: Union[Granularity, str, None],
+        group_size: Optional[int],
+    ) -> Granularity:
+        """
+        Parse the `Granularity` represented in the args.
+
+        Granularity can be specified in one of three ways:
+            1) `Granularity` object: one of PerToken(), PerAxis(), and PerGroup(group_size)
+            2) str: one of 'per_token', 'per_channel', and 'per_group'
+            3) None: `group_size` must be set instead, represents per group granularity
+        """
+        # If group_size is set, then granularity must be either "per_group" or None
+        if group_size is not None and granularity != "per_group" and granularity is not None:
+            raise ValueError("`group_size` conflicts with granularity '%s'" % granularity)
+
+        # Case 1: Granularity object
+        if isinstance(granularity, Granularity):
+            if not isinstance(granularity, (PerToken, PerAxis, PerGroup)):
+                raise ValueError("Granularity '%s' is not supported" % granularity)
+            if isinstance(granularity, PerAxis) and granularity.axis != 0:
+                raise ValueError("Only axis=0 is supported for PerAxis granularity")
+            return granularity
+
+        # Case 2: str granularity
+        if granularity == "per_token":
+            return PerToken()
+        elif granularity == "per_channel":
+            return PerAxis(axis=0)
+        elif granularity == "per_group":
+            if group_size is None:
+                raise ValueError("Granularity was 'per_group' but no `group_size` was set")
+            return PerGroup(group_size)
+        elif isinstance(granularity, str):
+            raise ValueError(
+                "Unexpected granularity: '%s', must be one of %s" %
+               (granularity, ["per_token", "per_channel", "per_group"])
+            )
+
+        # Case 3: None granularity + group_size was specified
+        if granularity is not None:
+            raise ValueError(
+                "Granularity '%s' has unexpected type %s" % (granularity, type(granularity))
+            )
+        if group_size is None:
+            raise ValueError("At least one of `granularity` or `group_size` must be set")
+        return PerGroup(group_size)
+
+    def _get_mapping_type(
+        self,
+        mapping_type: Optional[MappingType],
+        is_symmetric: Optional[bool],
+    ) -> MappingType:
+        """
+        Parse the `MappingType` represented in the args.
+
+        Mapping type can be specified in one of two ways:
+            1): `MappingType` object: one of SYMMETRIC or ASYMMETRIC
+            2): is_symmetric bool
+        """
+        if mapping_type is not None and is_symmetric is not None:
+            raise ValueError("Cannot set both `mapping_type` and `is_symmetric`")
+
+        # Case 0: Default to symmetric
+        if mapping_type is None and is_symmetric is None:
+            return MappingType.SYMMETRIC
+
+        # Case 1: MappingType object
+        if mapping_type is not None:
+            if mapping_type not in [MappingType.SYMMETRIC, MappingType.ASYMMETRIC]:
+                raise ValueError("MappingType '%s' is not supported" % mapping_type)
+            return mapping_type
+
+        # Case 2: is_symmetric flag
+        assert is_symmetric is not None
+        if is_symmetric:
+            return MappingType.SYMMETRIC
+        else:
+            return MappingType.ASYMMETRIC
+
+    @property
+    def group_size(self) -> int:
+        """
+        If this is per group granularity, return the group size.
+        Otherwise, throw an error.
+        """
+        if isinstance(self.granularity, PerGroup):
+            return self.granularity.group_size
+        else:
+            raise ValueError("`group_size` is undefined for %s granularity" % self.granularity)
+
+    @property
+    def is_symmetric(self) -> bool:
+        """
+        Return True if mapping type is symmetric, else False (asymmetric).
+        """
+        return self.mapping_type == MappingType.SYMMETRIC
+
+    def __setattr__(self, name: str, value: Any):
+        """
+        Support setting `group_size` and `is_symmetric`.
+        """
+        if name == "group_size":
+            super().__setattr__("granularity", PerGroup(value))
+        elif name == "is_symmetric":
+            mapping_type = MappingType.SYMMETRIC if value else MappingType.ASYMMETRIC
+            super().__setattr__("mapping_type", mapping_type)
+        else:
+            super().__setattr__(name, value)
 
 
 class ComposableQATQuantizer(TwoStepQuantizer):