Move decode context parallel validationn to `ParallelConfig` (#33239)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

vllm/config/parallel.py

@@ -348,6 +348,17 @@ class ParallelConfig:
                     "num_redundant_experts."
                 )
 
+        # Note(hc): In the current implementation of decode context
+        # parallel(DCP), tp_size needs to be divisible by dcp_size,
+        # because the world size does not change by dcp, it simply
+        # reuses the GPUs of TP group, and split one TP group into
+        # tp_size//dcp_size DCP groups.
+        if self.tensor_parallel_size % self.decode_context_parallel_size != 0:
+            raise ValueError(
+                f"tp_size={self.tensor_parallel_size} must be divisible by"
+                f"dcp_size={self.decode_context_parallel_size}."
+            )
+
         return self
 
     @property

vllm/engine/arg_utils.py

@@ -1389,16 +1389,6 @@ class EngineArgs:
             # global layers in interleaved sliding window models.
             sliding_window = model_config.get_sliding_window()
 
-        # Note(hc): In the current implementation of decode context
-        # parallel(DCP), tp_size needs to be divisible by dcp_size,
-        # because the world size does not change by dcp, it simply
-        # reuses the GPUs of TP group, and split one TP group into
-        # tp_size//dcp_size DCP groups.
-        assert self.tensor_parallel_size % self.decode_context_parallel_size == 0, (
-            f"tp_size={self.tensor_parallel_size} must be divisible by"
-            f"dcp_size={self.decode_context_parallel_size}."
-        )
-
         # Resolve "auto" kv_cache_dtype to actual value from model config
         resolved_cache_dtype = resolve_kv_cache_dtype_string(
             self.kv_cache_dtype, model_config