Fix name when saving text inversion embeddings in dreambooth advanced scripts

[DN6]

What does this PR do?

Textual Inversion Embeddings were being saved using f"{output_dir}/{args.output_dir} which can give weird paths when using relative path names. Updating to use just the directory name with Path. e.g. f"{args.output_dir}/{Path(args.output_dir).name}

Fixes # (issue)
#8909

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[linoytsaban]

Thanks!