Minor fixes to Textual Inversion Loader Mixin

[EandrewJones]

I went through and helped address most of the low hanging fruit as far as reviewer requests go.

Changes

• Renamed mixin
• Moved mixin from src/diffusers/textual_inversion_utils.py to src/diffusers/loaders.py
• Renamed the embeddings argument on the loading method to clarify that it's a path to embedding files, not embeddings, to clear up confusion about double loading
• Removed the unneeded class variable
• Added checks on the mixin to ensure the text_encoder and tokenizer exist
• Standardized types and documentation

Remaining fixes needed

• The ability to handle safe tensors
• The ability to load directly from the concepts library

I'm happy to help with either of the above two, but they're a little more complicated and probably require some coordination. Let me know how I can help.