[feat]: make DCP error msg clearer (#28443)

Signed-off-by: vnadathur <glvikramn@gmail.com>
Signed-off-by: WorldExplored <srreyansh.sethi@gmail.com>
Signed-off-by: Srreyansh Sethi <107075589+WorldExplored@users.noreply.github.com>
Co-authored-by: vnadathur <glvikramn@gmail.com>
Co-authored-by: vnadathur <236933696+vnadathur@users.noreply.github.com>

vllm/v1/worker/cp_utils.py

@@ -29,10 +29,11 @@ def check_attention_cp_compatibility(vllm_config: VllmConfig) -> None:
                 )
             if dcp_size > 1:
                 assert layer_impl.need_to_return_lse_for_decode, (
-                    "DCP requires attention impls to return"
-                    " the softmax lse for decode, but the impl "
-                    f"{layer_impl.__class__.__name__} "
-                    "does not return the softmax lse for decode."
+                    "Decode Context Parallelism (DCP) requires attention "
+                    "implementations to return the softmax LSE during decode, "
+                    f"but {layer_impl.__class__.__name__} does not. "
+                    "Try a different backend by setting "
+                    "VLLM_ATTENTION_BACKEND or disable DCP."
                 )
 
             if pcp_size > 1: