[DiffusionPipeline] Deprecate not throwing error when loading non-existant variant (#4011)

* Deprecate variant nicely

* make style

* Apply suggestions from code review
Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>

* Apply suggestions from code review
Co-authored-by: Pedro Cuenca <pedro@huggingface.co>

---------
Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>
Co-authored-by: Pedro Cuenca <pedro@huggingface.co>

src/diffusers/pipelines/pipeline_utils.py

@@ -1213,6 +1213,15 @@ class DiffusionPipeline(ConfigMixin):
             filenames = {sibling.rfilename for sibling in info.siblings}
             model_filenames, variant_filenames = variant_compatible_siblings(filenames, variant=variant)
 
+            if len(variant_filenames) == 0 and variant is not None:
+                deprecation_message = (
+                    f"You are trying to load the model files of the `variant={variant}`, but no such modeling files are available."
+                    f"The default model files: {model_filenames} will be loaded instead. Make sure to not load from `variant={variant}`"
+                    "if such variant modeling files are not available. Doing so will lead to an error in v0.22.0 as defaulting to non-variant"
+                    "modeling files is deprecated."
+                )
+                deprecate("no variant default", "0.22.0", deprecation_message, standard_warn=False)
+
             # remove ignored filenames
             model_filenames = set(model_filenames) - set(ignore_filenames)
             variant_filenames = set(variant_filenames) - set(ignore_filenames)

tests/pipelines/test_pipelines.py

@@ -14,6 +14,7 @@
 # limitations under the License.
 
 import gc
+import glob
 import json
 import os
 import random
@@ -56,6 +57,7 @@ from diffusers import (
     UniPCMultistepScheduler,
     logging,
 )
+from diffusers.pipelines.pipeline_utils import variant_compatible_siblings
 from diffusers.schedulers.scheduling_utils import SCHEDULER_CONFIG_NAME
 from diffusers.utils import (
     CONFIG_NAME,
@@ -1361,6 +1363,29 @@ class PipelineFastTests(unittest.TestCase):
             assert sd.config.safety_checker != (None, None)
             assert sd.config.feature_extractor != (None, None)
 
+    def test_warning_no_variant_available(self):
+        variant = "fp16"
+        with self.assertWarns(FutureWarning) as warning_context:
+            cached_folder = StableDiffusionPipeline.download(
+                "hf-internal-testing/diffusers-stable-diffusion-tiny-all", variant=variant
+            )
+
+        assert "but no such modeling files are available" in str(warning_context.warning)
+        assert variant in str(warning_context.warning)
+
+        def get_all_filenames(directory):
+            filenames = glob.glob(directory + "/**", recursive=True)
+            filenames = [f for f in filenames if os.path.isfile(f)]
+            return filenames
+
+        filenames = get_all_filenames(str(cached_folder))
+
+        all_model_files, variant_model_files = variant_compatible_siblings(filenames, variant=variant)
+
+        # make sure that none of the model names are variant model names
+        assert len(variant_model_files) == 0
+        assert len(all_model_files) > 0
+
 
 @slow
 @require_torch_gpu