[Spec Decoding] Use target model max length as default for draft model (

vllm-project#7706)

vllm/config.py

@@ -127,6 +127,7 @@ def __init__(
         rope_theta: Optional[float] = None,
         tokenizer_revision: Optional[str] = None,
         max_model_len: Optional[int] = None,
+        spec_target_max_model_len: Optional[int] = None,
         quantization: Optional[str] = None,
         quantization_param_path: Optional[str] = None,
         enforce_eager: Optional[bool] = None,
@@ -210,7 +211,8 @@ def __init__(
             hf_config=self.hf_text_config,
             max_model_len=max_model_len,
             disable_sliding_window=self.disable_sliding_window,
-            sliding_window_len=self.get_hf_config_sliding_window())
+            sliding_window_len=self.get_hf_config_sliding_window(),
+            spec_target_max_model_len=spec_target_max_model_len)
         self.served_model_name = get_served_model_name(model,
                                                        served_model_name)
         self.multimodal_config = self._init_multimodal_config(
@@ -1134,6 +1136,7 @@ def maybe_create_spec_config(
                 code_revision=draft_code_revision,
                 tokenizer_revision=target_model_config.tokenizer_revision,
                 max_model_len=None,
+                spec_target_max_model_len=target_model_config.max_model_len,
                 quantization=draft_quantization,
                 enforce_eager=target_model_config.enforce_eager,
                 max_seq_len_to_capture=target_model_config.
@@ -1563,6 +1566,7 @@ def _get_and_verify_max_len(
     max_model_len: Optional[int],
     disable_sliding_window: bool,
     sliding_window_len: Optional[int],
+    spec_target_max_model_len: Optional[int] = None,
 ) -> int:
     """Get and verify the model's maximum length."""
     derived_max_model_len = float("inf")
@@ -1605,6 +1609,11 @@ def _get_and_verify_max_len(
             # If max_model_len is specified, we use it.
             return max_model_len
 
+        if spec_target_max_model_len is not None:
+            # If this is a speculative draft model, we use the max model len
+            # from the target model.
+            return spec_target_max_model_len
+
         default_max_len = 2048
         logger.warning(
             "The model's config.json does not contain any of the following "