`from_single_file()` support missing/incomplete for Ideogram4 and Krea2 transformers

[dxqb]

Describe the bug

Neither Ideogram4Transformer2DModel nor Krea2Transformer2DModel currently support from_single_file(), though
for different reasons.

Ideogram4Transformer2DModel inherits FromOriginalModelMixin (since it was added in #13859), but it was never
added to SINGLE_FILE_LOADABLE_CLASSES in src/diffusers/loaders/single_file_model.py, and no
convert_ideogram4_transformer_checkpoint_to_diffusers-style mapping function exists in single_file_utils.py.
Calling from_single_file() on it currently raises unconditionally, since
_get_single_file_loadable_mapping_class() can only match classes that are registered in
SINGLE_FILE_LOADABLE_CLASSES:

For comparison, other recently added transformers (Cosmos, Flux2, ZImage, ...) both inherit the mixin and have a
checkpoint_mapping_fn registered.

Krea2Transformer2DModel doesn't inherit FromOriginalModelMixin at all, so from_single_file isn't even
defined on the class. This looks like it may simply be because there's no public checkpoint (original or
diffusers-format) to convert from yet — the docs mention loading "from a local directory produced by the Krea 2

Drafted by Claude, but confirmed, except the internals above

Reproduction

from diffusers import Ideogram4Transformer2DModel

Ideogram4Transformer2DModel.from_single_file("some/checkpoint.safetensors")
# ValueError: ...

Logs

System Info

cbdb637

Who can help?

Questions on single file checkpoints: @DN6

[AnkitNakhawa]

I’d be happy to work on the Ideogram4 portion of this issue. My plan is to identify the expected original checkpoint format, add the single-file loader registration and checkpoint conversion logic, and include focused tests for the mapping.

I’m also open to covering Krea2. Is there a public original or single-file Krea2 checkpoint format that from_single_file() is expected to support? If so, I can include that portion as well.