修改MLA prefill阶段出现的Device2Host拷贝同步现象。

vllm/attention/backends/mla/common.py

@@ -1072,6 +1072,8 @@ class MLACommonImpl(MLAAttentionImpl[T], Generic[T]):
         
         self.use_llama_nn = os.environ.get('LLAMA_NN') == '1'
 
+        self.has_context_default = os.environ.get('VLLM_HAS_CONTEXT_DEFAULT') == '1'
+
         # For MLA the v head dim is smaller than qk head dim so we pad out
         # v with 0s to match the qk head dim for attention backends that do
         # not support different headdims