[Frontend] Warn if user max_model_len is greater than derived max_model_len (#7080)

Signed-off-by: Jefferson Fialho <jfialho@ibm.com>
Co-authored-by: Nick Hill <nickhill@us.ibm.com>

vllm/envs.py

@@ -50,6 +50,7 @@ if TYPE_CHECKING:
     VLLM_NO_DEPRECATION_WARNING: bool = False
     CMAKE_BUILD_TYPE: Optional[str] = None
     VERBOSE: bool = False
+    VLLM_ALLOW_LONG_MAX_MODEL_LEN: bool = False
 
 
 def get_default_cache_root():
@@ -331,6 +332,15 @@ environment_variables: Dict[str, Callable[[], Any]] = {
     # If set, vllm will skip the deprecation warnings.
     "VLLM_NO_DEPRECATION_WARNING":
     lambda: bool(int(os.getenv("VLLM_NO_DEPRECATION_WARNING", "0"))),
+
+    # If the env var VLLM_ALLOW_LONG_MAX_MODEL_LEN is set, it allows
+    # the user to specify a max sequence length greater than
+    # the max length derived from the model's config.json.
+    # To enable this, set VLLM_ALLOW_LONG_MAX_MODEL_LEN=1.
+    "VLLM_ALLOW_LONG_MAX_MODEL_LEN":
+    lambda:
+    (os.environ.get("VLLM_ALLOW_LONG_MAX_MODEL_LEN", "0").strip().lower() in
+     ("1", "true")),
 }
 
 # end-env-vars-definition