Fix breaking error: "ort is not defined" (#626)

Fix "ort is not defined" issue.

Author: pcuenca

src/diffusers/onnx_utils.py

@@ -114,7 +114,7 @@ def _from_pretrained(
         cache_dir: Optional[str] = None,
         file_name: Optional[str] = None,
         provider: Optional[str] = None,
-        sess_options: Optional[ort.SessionOptions] = None,
+        sess_options: Optional["ort.SessionOptions"] = None,
         **kwargs,
     ):
         """