fix: draft model IMA by overide max_positional_embeddings (#10787)

Co-authored-by: Qiaolin Yu <qy254@cornell.edu>

python/sglang/srt/configs/model_config.py

@@ -210,6 +210,11 @@ class ModelConfig:
                 ):
                     logger.warning(msg)
                     self.context_len = context_length
+                    if is_draft_model:
+                        self.hf_text_config.max_position_embeddings = context_length
+                        logger.warning(
+                            f"Overriding the draft model's max_position_embeddings to {context_length}."
+                        )
                 else:
                     raise ValueError(
                         f"{msg} To allow overriding this maximum, set the env var SGLANG_ALLOW_OVERWRITE_LONGER_CONTEXT_LEN=1"