[Bugfix][Quantization] Don't reject fp8_e5m2 KV cache for non-fp8 quantized checkpoints (#45040)

Signed-off-by: Ting Sun <suntcrick@gmail.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

vllm/model_executor/layers/attention/attention.py

@@ -1,7 +1,7 @@
 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
-from typing import TYPE_CHECKING, Any
+from typing import TYPE_CHECKING, Any, cast
 
 import torch
 import torch.nn as nn
@@ -166,7 +166,21 @@ def _init_kv_cache_quant(
         # TODO (mgoin): kv cache dtype should be specified in the FP8
         # checkpoint config and become the "auto" behavior
         if layer.kv_cache_dtype == "fp8_e5m2":
-            raise ValueError("fp8_e5m2 kv-cache is not supported with fp8 checkpoints.")
+            # A compressed-tensors checkpoint stores fp8 KV scales only when it
+            # declares a kv_cache_scheme; weight-only ones declare none and must
+            # keep fp8_e5m2, the only fp8 KV dtype usable on Ampere.
+            from vllm.model_executor.layers.quantization.compressed_tensors.compressed_tensors import (  # noqa: E501
+                CompressedTensorsConfig,
+                CompressedTensorsKVCacheMethod,
+            )
+
+            if not isinstance(quant_method, CompressedTensorsKVCacheMethod) or (
+                cast(CompressedTensorsConfig, quant_method.quant_config).kv_cache_scheme
+                is not None
+            ):
+                raise ValueError(
+                    "fp8_e5m2 kv-cache is not supported with fp8 checkpoints."
+                )
         # If quantization is enabled, we make "k_scale" and "v_scale"
         # parameters so that it can be loaded from the model checkpoint.
         # The k/v_scale will then be converted back to native float32