Fix pydanitc create_model usage.

Fixes awslabs#767

The `model_name` argument was renamed to `__model_name` in
[1367](pydantic/pydantic#1367).

This change ensure that our code works with both future and past
versions of pydantic.

src/gluonts/core/component.py

@@ -389,15 +389,13 @@ def validator(init):
 
         if base_model is None:
             PydanticModel = create_model(
-                model_name=f"{init_clsnme}Model",
+                f"{init_clsnme}Model",
                 __config__=BaseValidatedInitializerModel.Config,
                 **init_fields,
             )
         else:
             PydanticModel = create_model(
-                model_name=f"{init_clsnme}Model",
-                __base__=base_model,
-                **init_fields,
+                f"{init_clsnme}Model", __base__=base_model, **init_fields,
             )
 
         def validated_repr(self) -> str: