[Bugfix] Fix extract_hidden_states crash with quantized KV cache dtype (#39160)

Signed-off-by: Yubo Wang <yubowang2019@gmail.com>

vllm/model_executor/models/extract_hidden_states.py

@@ -9,6 +9,7 @@ extract_hidden_states speculative decoding method.
 """
 
 from collections.abc import Iterable
+from dataclasses import replace
 from typing import ClassVar
 
 import torch
@@ -352,6 +353,10 @@ class ExtractHiddenStatesModel(nn.Module):
 
         cache_config = vllm_config.cache_config
 
+        # Hidden states dtype should be independent of KV cache dtype.
+        if cache_config is not None and is_quantized_kv_cache(cache_config.cache_dtype):
+            cache_config = replace(cache_config, cache_dtype="auto")
+
         # Create a single cache-only attention layer
         # Note: We set num_heads <- self.num_hidden_states
         # and head_size <- hidden_size so that we can insert