Revert AutoencoderKLWan's dim_mult default value back to list

[dg845]

What does this PR do?

This PR reverts the default value for the dim_mult argument to AutoencoderKLWan.__init__ back to a List (e.g. [1, 2, 4, 4] instead of (1, 2, 4, 4)). The rationale behind this is that some code of the Wan VAE code assumes that dim_mult is type List, such as

diffusers/src/diffusers/models/autoencoders/autoencoder_kl_wan.py

Line 542 in 093cd3f

dims = [dim * u for u in [1] + dim_mult]

This causes errors when dim_mult is not explicitly set, such as in convert_vae() in scripts/convert_wan_to_diffusers.py.

dim_mult defaulted to a List prior to #12544, but was incorrectly annotated as type Tuple[int], so I believe that PR inadvertently changed the default argument to a Tuple. I have changed the type annotation to List[int] in this PR.

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

@yiyixuxu

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