[Core] fix: legacy model mapping

[sayakpaul]

What does this PR do?

Otherwise, this would fail:

from diffusers import Transformer2DModel
import torch

weight_dtype = torch.float16

transformer = Transformer2DModel.from_pretrained(
    "PixArt-alpha/PixArt-Sigma-XL-2-1024-MS",
    subfolder='transformer',
    torch_dtype=weight_dtype,
    use_safetensors=True,
)

Can do a patch release for this.

[HuggingFaceDocBuilderDev]

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[yiyixuxu]

is this caught by any of our tests?

[sayakpaul]

No, we don’t check for subfolder tests from a pipeline checkpoint. Can add in a future PR for tiny checkpoints.

[sayakpaul]

Here you go: #8422