[UX] DCP Topology Validation (#49777)

Signed-off-by: Taneem Ibrahim <taneem.ibrahim@gmail.com>

vllm/config/model.py

@@ -1298,27 +1298,33 @@ class ModelConfig:
         decode_context_parallel_size = parallel_config.decode_context_parallel_size
         if decode_context_parallel_size > 1 and not self.use_mla:
             total_num_kv_heads = self.get_total_num_kv_heads()
-            assert tensor_parallel_size > total_num_kv_heads, (
-                f"tensor parallel size {tensor_parallel_size} must be greater "
-                f"than total num kv heads {total_num_kv_heads} when enable "
-                f"decode context parallel for GQA/MQA"
-            )
+            if tensor_parallel_size <= total_num_kv_heads:
+                raise ValueError(
+                    "Decode context parallelism for GQA/MQA requires "
+                    f"`--tensor-parallel-size` ({tensor_parallel_size}) to be "
+                    "greater than the model's total number of KV heads "
+                    f"({total_num_kv_heads}). Increase `--tensor-parallel-size` "
+                    "or set `--decode-context-parallel-size 1`."
+                )
 
             max_dcp_size = tensor_parallel_size // total_num_kv_heads
-            assert decode_context_parallel_size <= max_dcp_size, (
-                f"decode context parallel size must less than or equal to "
-                f"(tensor parallel size {tensor_parallel_size} // total "
-                f"num kv heads {total_num_kv_heads}) = {max_dcp_size}, "
-                f"but got {decode_context_parallel_size}"
-            )
+            if decode_context_parallel_size > max_dcp_size:
+                raise ValueError(
+                    "`--decode-context-parallel-size` "
+                    f"({decode_context_parallel_size}) exceeds the maximum "
+                    f"supported value ({max_dcp_size}) for "
+                    f"`--tensor-parallel-size` ({tensor_parallel_size}) and "
+                    f"{total_num_kv_heads} model KV heads."
+                )
 
             num_q_per_kv = total_num_attention_heads // total_num_kv_heads
-            assert num_q_per_kv % decode_context_parallel_size == 0, (
-                f"Total number of q per kv attn heads ({num_q_per_kv})"
-                " must be divisible by dcp world size when enable "
-                "decode context parallel for GQA "
-                f"({parallel_config.decode_context_parallel_size})."
-            )
+            if num_q_per_kv % decode_context_parallel_size != 0:
+                raise ValueError(
+                    "The model's number of query heads per KV head "
+                    f"({num_q_per_kv}) must be divisible by "
+                    "`--decode-context-parallel-size` "
+                    f"({decode_context_parallel_size}) for GQA/MQA."
+                )
 
         # torch_shm uses a single IPC queue to rank 0; DP>1 is
         # incompatible because API servers can't know which