[MInimax H3] Fix VAE decode (#14754)

* intiial fix

* fix code quality

* fix failing pr test

* fix gpu test

* don't override the requested dtype

* add test

* apply review

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Co-authored-by: YiYi Xu <yixu310@gmail.com>
Co-authored-by: Dhruv Nair <dhruv.nair@gmail.com>