Add from_single_file support to ErnieImageTransformer2DModel

[akshan-main]

What does this PR do?

Fixes #13722. ErnieImageTransformer2DModel does not implement from_single_file, unlike other models (Flux, Chroma, Z-Image, etc.), so loading a single-file checkpoint (e.g. a OneTrainer finetune) errors with AttributeError.

Adding standard FromOriginalModelMixin to the class and register a minimal checkpoint converter that strips the common model.diffusion_model. prefix(same shape as the Flux/Chroma converters)
Fixes #13722

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline?
• Did you read our philosophy doc (important for complex PRs)?
• Was this discussed/approved via a GitHub issue? ErnieImageTransformer2DModel does not implement from_single_file #13722
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

Who can review?

@DN6 @sayakpaul

[wadeKeith]

Clean addition - from_single_file support for ErnieImageTransformer2DModel follows existing patterns. LGTM! Reviewed by Hermes Agent.

[DN6]

@akshan-main do you have an example snippet to verify that we can load the model?

[akshan-main]

@DN6 there's no public single-file ernie checkpoint yet, so I verified by saving the sharded baidu/ERNIE-Image transformer to a single safetensors file and then loading it back via from_single_file. Colab output with the PR's patches applied on top of main: #13722 (comment)

Output:

loading sharded checkpoint...
loaded, params: 8033490048
saved: /content/ernie_transformer_single.safetensors
loading via from_single_file...
loaded, params: 8033490048

key match: True
sample tensor 'x_embedder.proj.weight' equal: True

ROUND-TRIP OK