Pyserini: Port Torch Models to MLX and enable Indexing and Searching on Mac M series machines

[ToluClassics]

Summary: The goal is to extend the encode module (https://github.com/castorini/pyserini/tree/master/pyserini/encode ) of Pyserini to allow loading and inferencing retriever models in MLX similar to the way it is currently done in PyTorch. This enables us to index and search effectively on Mac M series laptops with near Cuda speed. So similar to DprDocumentEncoder we will have something like MlxDprDocumentEncoder .

What this entails is translating model architecture code from PyTorch to MLX. The interface of both frameworks are very similar except that MLX is more numpy like