Allow custom model names for Dense, Sparse embeddings, fix Sparse Embeddings

[tomaarsen]

Hello!

Pull Request overview

• Allow custom model_name in DefaultLocalSparseEmbedding and DefaultLocalDenseEmbedding
• Simplify model loading, only 1 _get_model across DenseEmbedding, SparseEmbedding, and ReRanker via a _get_model_class property
• Fix SparseEncoder model loading: It used SentenceTransformer previously. Also, encode_query & encode_document exists for both SparseEncoder and SentenceTransformer.
• Removed _manual_sparse_encode for older Sentence Transformer: I would recommend requiring at least Sentence Transformers v5.0 so the SparseEncoder can be used.
• Replaced SentenceTransformerReRanker with DefaultLocalReRanker in the docs, as the previous doesn't exist anymore.

Details:

This PR extends the support for local dense, sparse, and reranker models a good bit. I've had some installation issues, so I'm unable to properly run the tests at the moment, but you should get much clearer results for the SparseEmbedding class. I also simplified the sparse embedding post-processing using model.decode, which also removes some issues with SparseCUDA and SparseCPU not implementing all operations.

Please do let me know if you have any questions.

• Tom Aarsen

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