Fix ignore warning

See https://github.com/THUDM/ChatGLM2-6B/issues/639

modeling_chatglm.py

@@ -1107,7 +1107,7 @@ class ChatGLMForConditionalGeneration(ChatGLMPreTrainedModel):
             )
         elif generation_config.max_new_tokens is not None:
             generation_config.max_length = generation_config.max_new_tokens + input_ids_seq_length
-            if not has_default_max_length:
+            if not has_default_max_length and generation_config.max_length is not None:
                 logger.warn(
                     f"Both `max_new_tokens` (={generation_config.max_new_tokens}) and `max_length`(="
                     f"{generation_config.max_length}) seem to have been set. `max_new_tokens` will take precedence. "