Add unload_textual_inversion method (#6656)
* Add unload_textual_inversion
* Fix dicts in tokenizer
* Fix quality
* Apply suggestions from code review
Co-authored-by: YiYi Xu <yixu310@gmail.com>
* Fix variable name after last update
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Co-authored-by: YiYi Xu <yixu310@gmail.com>
