[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>
