aphrodite-engine/aphrodite/quantization/kv_cache.py

import torch

from aphrodite.common.utils import print_warning_once
from aphrodite.quantization.base_config import (QuantizationConfig,
                                                QuantizeMethodBase)


class BaseKVCacheMethod(QuantizeMethodBase):
    """
    Quant method that adds `_k_scale` and `_v_scale` attributes to the
    Attention layer to support loading those scaling factors from checkpoints. 
    The k/v_scale will be used to:
        - quantize k/v_cache entries before saving them to the cache
        - dequantize k/v_cache entries before fetching them from the cache
    :param quant_config: the appropriate QuantizationConfig 
    """

    def __init__(self, quant_config: QuantizationConfig):
        self.quant_config = quant_config

    def create_weights(self, layer: torch.nn.Module):
        """
        Create "weight" (aka k_scale and v_scale) for an attention layer.
        """
        # Initialize the KV cache scales to -1.0, which is an invalid value.
        # If the k/v_scale appears in the checkpoint, it will be
        # overwritten when loading weights.
        layer.k_scale = torch.nn.Parameter(torch.tensor(-1.0),
                                           requires_grad=False)
        layer.v_scale = torch.nn.Parameter(torch.tensor(-1.0),
                                           requires_grad=False)

    def apply(self, layer: torch.nn.Module) -> torch.Tensor:
        raise RuntimeError(
            f"{self.__class__.__name__}.apply should not be called.")

    def process_weights_after_loading(self, layer: torch.nn.Module) -> None:
        # If the kv-cache dtype is auto, we enforce the k/v_scale to be 1.0
        # regardless whether the kv-scale is available in the checkpoint.
        if layer.kv_cache_dtype != "auto":
            if layer.k_scale > 0.0 and layer.v_scale > 0.0:
                # We prefer to use separate k_scale and v_scale if present
                k_scale = layer.k_scale.to("cpu").tolist()
                v_scale = layer.v_scale.to("cpu").tolist()
            elif layer.k_scale < 0.0 and layer.v_scale < 0.0:
                # If no scales were loaded (both scales are invalid negative
                # values), use the default value of 1.0
                k_scale = 1.0
                v_scale = 1.0
            else:
                # If we find a single kv_scale in the checkpoint, we remap
                # kv_scale to k_scale during weight loading, and duplicate
                # k_scale to v_scale here
                assert layer.k_scale > 0.0
                scale_to_duplicate = max(layer.k_scale, layer.v_scale)
                k_scale = scale_to_duplicate.to("cpu").tolist()
                v_scale = scale_to_duplicate.to("cpu").tolist()

            if not isinstance(k_scale, float) or not isinstance(
                    v_scale, float):
                raise ValueError("Only support per-tensor scaling factor "
                                 "for fp8 KV cache")

            # These are used in the final Attention.forward()
            layer._k_scale = k_scale
            layer._v_scale = v_scale
            if (layer._k_scale == 1.0 and layer._v_scale == 1.0
                    and "e5m2" not in layer.kv_cache_dtype):
                print_warning_once(
                    "Using KV cache scaling factor 1.0 for fp8_e4m3. This "
                    "may cause accuracy issues. Please make sure k/v_scale "
                    "scaling factors are available in the fp8 checkpoint.")

        del layer.k_scale
        del layer.v_scale