feat: implements vectorised sparse BM25 encoder

fix: catch RuntimeError for missing torch import

chore: update pyproject

chore: fix numerical inconsistencies

test: add tests for tokenizers

chore: fit sparse encoders after route modifications in HybridRouter

fix: remove decorator around the HybridRouter.__init__

fix: give the functional BM25 a distinct filename

fix: fix route fitting for add/strategy

fix: _async_encode should take a sparse encoding function

fix: empty sparse embeddings

fix: _encode / _async_encode match superclass signature