Support saving multiple t2i adapter models under one checkpoint (#4798)
* adding save and load for MultiAdapter, adding test
* Apply suggestions from code review
Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>
* Adding changes from review test_stable_diffusion_adapter
* import sorting fix
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Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>