[MISC] add cpu_kvcache_space_bytes to CacheConfig (#19812)

Signed-off-by: Andy Xie <andy.xning@gmail.com>

vllm/config.py

@@ -1506,6 +1506,8 @@ class CacheConfig:
     """This enables dynamic calculation of `k_scale` and `v_scale` when
     kv_cache_dtype is fp8. If `False`, the scales will be loaded from the model
     checkpoint if available. Otherwise, the scales will default to 1.0."""
+    cpu_kvcache_space_bytes: Optional[int] = None
+    """(CPU backend only) CPU key-value cache space."""
 
     # Will be set after profiling.
     num_gpu_blocks: Optional[int] = field(default=None, init=False)