[Fix] Add model sequence length into model config (#575)

vllm/config.py

@@ -109,6 +109,26 @@ class ModelConfig:
         total_num_attention_heads = self.hf_config.num_attention_heads
         return total_num_attention_heads // parallel_config.tensor_parallel_size
 
+    def get_max_model_len(self) -> int:
+        max_model_len = float("inf")
+        possible_keys = [
+            # OPT
+            "max_position_embeddings",
+            # GPT-2
+            "n_positions",
+            # MPT
+            "max_seq_len",
+            # Others
+            "max_sequence_length",
+            "max_seq_length",
+            "seq_len",
+        ]
+        for key in possible_keys:
+            max_len_key = getattr(self.hf_config, key, None)
+            if max_len_key is not None:
+                max_model_len = min(max_model_len, max_len_key)
+        return max_model_len
+
     def get_num_layers(self, parallel_config: "ParallelConfig") -> int:
         total_num_hidden_layers = self.hf_config.num_hidden_layers
         return total_num_hidden_layers // parallel_config.pipeline_parallel_size