add deprecation warning.

Author: sayakpaul

src/diffusers/models/modeling_flax_utils.py

@@ -290,6 +290,10 @@ def from_pretrained(
         You should probably TRAIN this model on a down-stream task to be able to use it for predictions and inference.
         ```
         """
+        logger.warning(
+            "Flax classes are deprecated and will be removed in Diffusers v1. We "
+            "recommend migrating to PyTorch classes or pinning your version of Diffusers."
+        )
         config = kwargs.pop("config", None)
         cache_dir = kwargs.pop("cache_dir", None)
         force_download = kwargs.pop("force_download", False)