`ModelHubMixin`: more metadata + arbitrary config types + proper guide (#2230)
* Configurable modelcard in ModelHubMixin
* support encoders/decoders for custom args in ModelHubMixin
* document mixin
* more docs
* ocs
* doc
* Apply suggestions from code review
Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>
* move up
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Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>