update saving and loading methods for faiss index so to accept path l…
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Seems ok for me, what do you think @lhoestq ?
- Update saving and loading methods for faiss index so to accept path like objects from pathlib

The current code only supports using a string type to save and load a faiss index. This change makes it possible to use a string type OR a Path from [pathlib](https://docs.python.org/3/library/pathlib.html). The codes becomes a more intuitive this way I think.
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