add PreTrainedTokenizerFast in loadable classes

src/diffusers/pipeline_utils.py

@@ -40,6 +40,7 @@ LOADABLE_CLASSES = {
     },
     "transformers": {
         "PreTrainedTokenizer": ["save_pretrained", "from_pretrained"],
+        "PreTrainedTokenizerFast": ["save_pretrained", "from_pretrained"],
         "PreTrainedModel": ["save_pretrained", "from_pretrained"],
     },
 }