Restore `is_modelcards_available` in `.utils` (#224)

Restore `is_modelcards_available` in `.utils`.

Otherwise attempting to import `hub_utils` (in training scripts, for
example), fails.

This was removed during the refactor in df90f0ce.

src/diffusers/utils/__init__.py

@@ -33,6 +33,7 @@ from .import_utils import (
     is_torch_available,
     is_transformers_available,
     is_unidecode_available,
+    is_modelcards_available,
     requires_backends,
 )
 from .logging import get_logger