[Modular] Fix dtype assignment when type hint is AutoModel

[DN6]

What does this PR do?

Oversight on my part when adding #13168. We avoid passing torch_dtype to non nn.Modules such as Processors and Tokenizers because these objects do not accept that anymore and instead store it in init_kwargs which then throw an error during serialization. However, I neglected the fact that AutoModel does not subclass ModelMixin, so if type_hint is set to diffusers AutoModel the check evaluates to True and we skip passing torch_dtype

This PR fixes the issue by checking whether type_hint is AutoModel

Fixes # (issue)

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[DN6]

Moved this up because now we need to import AutoModel regardless of whether type_hint is provided or not.

[HuggingFaceDocBuilderDev]

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[yiyixuxu]

thanks!