fix: remove torch_dtype="auto" option from docstrings

[johannaSommer]

What does this PR do?

This PR removes the option of specifying dtype="auto" from several Mixins and Pipeline classes.

I've removed the description of this possibility from the docstrings, this option is not supported in the code. Additionally, I've removed the type hint str from the torch_dtype keyword argument, as this is expected to be either a torch.dtype or a dict.

Fixes #11432

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@DN6

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