Checking changes for 0.18 release.

[owenvallis]

• Rerunning all Notebooks
• Refactor setup to make as many deps optional as possible [Note: might come back to this if it's awkward to run the hello world]
• Clean up types and try to increase consistency between the different Search and Store classes
• Shorten / clean up the module names in Search and Store
• Implement lazy loading for all Search and Store modules to avoid having to load the deps.
• Fix and update tests
• Migrate to using pathlib where possible
• Other small fixes and updates.

[review-notebook-app[bot]]

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[owenvallis]

@ebursztein and @dnazila, I made some headway on the 0.18 changes. It's in a good place (thanks for the new backends Ali) and I just need to run the unsupervised and multimodal notebooks to make sure they are good to go. I do have a few questions:

1) I feel like the API for the Search modules could be tightened up. The Faiss index has a training build method and this means that you wouldn't use the build param in the add methods like we do with the other Search modules. Also, the notion of "is_built" seems to mean slightly different things for each, e.g., Linear Search is always built and ready... it just might be empty. 2) What do you think about pulling out the deps into the extra_requires? This will make the main install much smaller, but running the hello world notebooks likely requires all the visualize deps.

Also looks like some test issues between TF versions. I'll try and sort that out as well.

[owenvallis]

@ebursztein I still need to update the other notebooks, but feel free to take a look at the changes to the modules if you want. I think those should be good to go.