[attention][DCP] use AttentionImpl.need_to_return_lse_for_decode (#24372)

Signed-off-by: youkaichao <youkaichao@gmail.com>

vllm/v1/attention/backends/mla/common.py

@@ -1592,10 +1592,6 @@ class MLACommonImpl(MLAAttentionImpl[M], Generic[M]):
 
             # recorect dcp attn_out with lse.
             if self.dcp_world_size > 1:
-                assert lse is not None, (
-                    "For a mla backend want to enable"
-                    "DCP, it is mandatory that the corresponding decode attn"
-                    "kernel return the softmax lse.")
                 attn_out = cp_lse_ag_out_rs(attn_out, lse, get_dcp_group())
 
             # v_up projection

vllm/v1/attention/backends/mla/flashmla.py

@@ -133,6 +133,8 @@ class FlashMLAMetadataBuilder(MLACommonMetadataBuilder[FlashMLAMetadata]):
 
 class FlashMLAImpl(MLACommonImpl[FlashMLAMetadata]):
 
+    can_return_lse_for_decode: bool = True
+
     def __init__(
             self,
             num_heads: int,