fix: ValueError when using FromOriginalModelMixin in subclasses #8440 (#8454)

* fix: ValueError when using FromOriginalModelMixin in subclasses #8440

(cherry picked from commit 92859978436acf844760fc0e992165b489d0180a)

* Update src/diffusers/loaders/single_file_model.py
Co-authored-by: Dhruv Nair <dhruv.nair@gmail.com>

* Update single_file_model.py

* Update single_file_model.py

---------
Co-authored-by: Dhruv Nair <dhruv.nair@gmail.com>
Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>

src/diffusers/loaders/single_file_model.py

@@ -11,6 +11,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+import importlib
 import inspect
 import re
 from contextlib import nullcontext
@@ -72,6 +73,17 @@ SINGLE_FILE_LOADABLE_CLASSES = {
 }
 
 
+def _get_single_file_loadable_mapping_class(cls):
+    diffusers_module = importlib.import_module(__name__.split(".")[0])
+    for loadable_class_str in SINGLE_FILE_LOADABLE_CLASSES:
+        loadable_class = getattr(diffusers_module, loadable_class_str)
+
+        if issubclass(cls, loadable_class):
+            return loadable_class_str
+
+    return None
+
+
 def _get_mapping_function_kwargs(mapping_fn, **kwargs):
     parameters = inspect.signature(mapping_fn).parameters
 
@@ -149,8 +161,9 @@ class FromOriginalModelMixin:
         ```
         """
 
-        class_name = cls.__name__
-        if class_name not in SINGLE_FILE_LOADABLE_CLASSES:
+        mapping_class_name = _get_single_file_loadable_mapping_class(cls)
+        # if class_name not in SINGLE_FILE_LOADABLE_CLASSES:
+        if mapping_class_name is None:
             raise ValueError(
                 f"FromOriginalModelMixin is currently only compatible with {', '.join(SINGLE_FILE_LOADABLE_CLASSES.keys())}"
             )
@@ -195,7 +208,7 @@ class FromOriginalModelMixin:
                 revision=revision,
             )
 
-        mapping_functions = SINGLE_FILE_LOADABLE_CLASSES[class_name]
+        mapping_functions = SINGLE_FILE_LOADABLE_CLASSES[mapping_class_name]
 
         checkpoint_mapping_fn = mapping_functions["checkpoint_mapping_fn"]
         if original_config:
@@ -207,7 +220,7 @@ class FromOriginalModelMixin:
             if config_mapping_fn is None:
                 raise ValueError(
                     (
-                        f"`original_config` has been provided for {class_name} but no mapping function"
+                        f"`original_config` has been provided for {mapping_class_name} but no mapping function"
                         "was found to convert the original config to a Diffusers config in"
                         "`diffusers.loaders.single_file_utils`"
                     )
@@ -267,7 +280,7 @@ class FromOriginalModelMixin:
         )
         if not diffusers_format_checkpoint:
             raise SingleFileComponentError(
-                f"Failed to load {class_name}. Weights for this component appear to be missing in the checkpoint."
+                f"Failed to load {mapping_class_name}. Weights for this component appear to be missing in the checkpoint."
             )
 
         ctx = init_empty_weights if is_accelerate_available() else nullcontext