fix: ValueError when using FromOriginalModelMixin in subclasses #8440

[luocfprime]

(cherry picked from commit 9285997)

What does this PR do?

Fixes # (issue)

#8440

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• Did you write any new necessary tests? test snippet below

import torch

from diffusers import AutoencoderKL


class MyVAE(AutoencoderKL):
    pass


if __name__ == "__main__":
    MyVAE.from_single_file("./checkpoints/realisticVisionV51_v51VAE.safetensors",
                           subfolder="vae",
                           torch_dtype=torch.float16,
                           use_safetensors=True)

    print("done.")

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@DN6

[luocfprime]

I made 2 commits to 1. resolve the review, 2. remove the redundant code fragments of the original code

[HuggingFaceDocBuilderDev]

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[DN6]

Hi @fkcptlst I will resolve the issues with the CI and merge this soon. Thanks for being patient.