add separate quantization primitives for float8

ghstack-source-id: b5340379b49bdab5e00e4c27d7444dc7d7f1acd7
ghstack-comment-id: 2608048970
Pull Request resolved: #1597

Author: danielvegamyhre

torchao/quantization/quant_primitives.py

@@ -5,22 +5,26 @@
 # LICENSE file in the root directory of this source tree.
 
 import math
-from enum import Enum, auto
+from enum import auto, Enum
 from typing import Callable, Dict, List, Optional, Tuple, Union
 
 import torch
+from torchao.float8.float8_utils import (
+    ScalingGranularity,
+    tensor_to_scale as tensor_to_float8_scale,
+)
 
 from torchao.prototype.custom_fp_utils import (
     _f32_to_floatx_unpacked,
     _floatx_unpacked_to_f32,
     _n_ones,
 )
 from torchao.utils import (
+    _is_float8_type,
+    _register_custom_op,
     TORCH_VERSION_AT_LEAST_2_3,
     TORCH_VERSION_AT_LEAST_2_5,
     TORCH_VERSION_AT_LEAST_2_6,
-    _is_float8_type,
-    _register_custom_op,
 )
 
 __all__ = [
@@ -39,6 +43,9 @@
     "MappingType",
     "ZeroPointDomain",
     "TorchAODType",
+    "choose_qparams_affine_float8",
+    "quantize_affine_float8",
+    "dequantize_affine_float8",
 ]
 
 
@@ -1300,3 +1307,65 @@ def dequantize_affine_floatx(
     tensor = tensor * scale.float().view(-1, 1)
     tensor = tensor.to(dtype=output_dtype)
     return tensor
+
+
+def choose_qparams_affine_float8(
+    tensor: torch.Tensor, float8_dtype: torch.dtype
+) -> torch.Tensor:
+    """
+    Calculates float8 scaling factor for the given high precision tensor, using tensorwise granularity.
+
+    Args:
+        tensor (torch.Tensor): Input tensor to be quantized.
+        float8_dtype (torch.dtype): Data type of the quantized tensor (e.g., torch.float8_e4m3fn, torch.float8_e5m2).
+    """
+    # NOTE: quantization primitives are hardcoded to use axiswise granularity w/ axis=1 right now:
+    # https://github.com/pytorch/ao/blob/5d1444bdef6df15eb89c4c5716ede1c5f8677798/torchao/dtypes/affine_quantized_tensor.py#L416
+    scale = tensor_to_float8_scale(
+        tensor,
+        float8_dtype,
+        scaling_granularity=ScalingGranularity.AXISWISE,
+        axiswise_dim=1,
+    )
+    return scale
+
+
+def quantize_affine_float8(
+    tensor: torch.Tensor,
+    scale: torch.Tensor,
+    float8_dtype: torch.dtype,
+) -> torch.Tensor:
+    """
+    Quantizes the high precision floating point tensor to a float8 tensor, using the given scaling factor.
+
+    Args:
+        tensor (torch.Tensor): Input tensor to be quantized.
+        scale (torch.Tensor): Scaling factor for the quantization.
+        float8_dtype (torch.dtype): Data type of the quantized tensor (e.g., torch.float8_e4m3fn, torch.float8_e5m2).
+    """
+    # Note: when the line below is compiled with `torch.compile`, `tensor` is automatically
+    # upcasted to `float32` to multiply with the scale
+    # In order to match numerics between eager and compile, we upcast manually here.
+    tensor_scaled = tensor.to(torch.float32) * scale
+    max_value = torch.finfo(float8_dtype).max
+    tensor_clamped = tensor_scaled.clamp(min=-max_value, max=max_value)
+    fp8_tensor = tensor_clamped.to(float8_dtype)
+    return fp8_tensor
+
+
+def dequantize_affine_float8(
+    tensor: torch.Tensor,
+    scale: torch.Tensor,
+    output_dtype: torch.dtype = torch.float32,
+) -> torch.Tensor:
+    """
+    Dequantizes the float8 tensor to float32 tensor.
+
+    Args:
+        tensor (torch.Tensor): Input float8 tensor to be dequantized.
+        scale (torch.Tensor): Scaling factor for the dequantization.
+        output_dtype (torch.dtype): Data type of the output tensor (e.g., torch.float32).
+    """
+    fp8_tensor = tensor.to(torch.float32)
+    hp_tensor = fp8_tensor / scale
+    return hp_tensor.to(output_dtype)