This PR adds support for tensor columns in the to_tf() and to_torch() APIs.
For Torch, this involves an explicit extension array check and (zero-copy) conversion of the tensor column to a NumPy array before converting the column to a Torch tensor.
For TensorFlow, this involves bypassing df.values when converting tensor feature columns to NumPy arrays, instead manually creating a single NumPy array from the column Series.
In both cases, I think that the UX around heterogeneous feature columns and squeezing the column dimension could be improved, but I'm saving that for a future PR.
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.
