This PR adds experimental support for random access to datasets. A Dataset can be random access enabled by calling `ds.to_random_access_dataset(key, num_workers=N)`. This creates a RandomAccessDataset.
RandomAccessDataset partitions the dataset across the cluster by the given sort key, providing efficient random access to records via binary search. A number of worker actors are created, each of which has zero-copy access to the underlying sorted data blocks of the Dataset.
Performance-wise, you can expect each worker to provide ~3000 records / second via ``get_async()``, and ~10000 records / second via ``multiget()``.
Since Ray actor calls go direct from worker->worker, throughput scales linearly with the number of workers.
This PR properly exposes `TableRow` as a public API (API docs + the "Public" tag), since it's already exposed to the user in our row-based ops. In addition, the following changes are made:
1. During row-based ops, we also choose a batch format that lines up with the current dataset format in order to eliminate unnecessary copies and type conversions.
2. `TableRow` now derives from `collections.abc.Mapping`, which lets `TableRow` better interop with code expecting a mapping, and includes a few helpful mixins so we only have to implement `__getitem__`, `__iter__`, and `__len__`.
Preview: [docs](https://ray--21931.org.readthedocs.build/en/21931/data/dataset.html)
The Ray Data project's docs now have a clearer structure and have partly been rewritten/modified. In particular we have
- [x] A Getting Started Guide
- [x] An explicit User / How-To Guide
- [x] A dedicated Key Concepts page
- [x] A consistent naming convention in `Ray Data` whenever is is referred to the project.
This surfaces quite clearly that, apart from the "Getting Started" sections, we really only have one real example. Once we have more, we can create an "Example" section like many other sub-projects have. This will be addressed in https://github.com/ray-project/ray/issues/21838.
This PR consolidates both #21667 and #21759 (look there for features), but improves on them in the following way:
- [x] we reverted renaming of existing projects `tune`, `rllib`, `train`, `cluster`, `serve`, `raysgd` and `data` so that links won't break. I think my consolidation efforts with the `ray-` prefix were a little overeager in that regard. It's better like this. Only the creation of `ray-core` was a necessity, and some files moved into the `rllib` folder, so that should be relatively benign.
- [x] Additionally, we added Algolia `docsearch`, screenshot below. This is _much_ better than our current search. Caveat: there's a sphinx dependency that needs to be replaced (`sphinx-tabs`) by another, newer one (`sphinx-panels`), as the former prevents loading of the `algolia.js` library. Will follow-up in the next PR (hoping this one doesn't get re-re-re-re-reverted).
