feat: Add encode and similarity of Sentence transformers

[JingyaHuang]

What does this PR do?

Fixes #975

• Add encode() and similarity()
• tests
• new doc for sentence transformers

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[michaelbenayoun]

Suggested change

	def similarity_fn_name(self) -> Literal["cosine", "dot", "euclidean", "manhattan"]:
	def similarity_fn_name(self) -> SimilarityFunction:

[JingyaHuang]

why? I think in the setter self._similarity will always be str.

[michaelbenayoun]

If self._similarity_fn_name is None, then you return SimilarityFunction.COSINE.
But that's a nit I guess, it's equivalent right?

[JingyaHuang]

Yeah in that case it returns SimilarityFunction.COSINE which is just the str "cosine" which is not a function but its name:
https://github.com/huggingface/sentence-transformers/blob/85ec64559f4414aa536eca4bf53538291e0a333f/sentence_transformers/similarity_functions.py#L31C14-L31C22