Make `pyarrow` optional
The whole datasets library is pretty much a wrapper to pyarrow (just take a look at some of the source for a Dataset) https://github.com/huggingface/datasets/blob/51aef08ad7053c0bfe8f9a961207b26df15850d3/src/datasets/arrow_dataset.py#L639 

I think removing the pyarrow dependency would involve a complete rewrite / a different library with minimal functionality (datasets-lite ?) Thanks for the proposal, @KOLANICH. And also thanks for your answer, @dconathan.

Indeed, we are using `pyarrow` as the backend for our datasets, in order to cache them and also allow memory-mapping (using datasets larger than your RAM memory).

One way to avoid using `pyarrow` could be loading the datasets in streaming mode, by passing `streaming=True` to `load_dataset`. This way you basically get a generator for the dataset; nothing is downloaded, nor cached.  Thanks for the info. Could `datasets` then be made optional for `transformers` instead? I used `transformers` only to deal with pretrained models to deploy them (convert to ONNX, and then I use TVM), so I don't really need `pyarrow` and `datasets` by now.

**Is your feature request related to a problem? Please describe.**
Is `pyarrow` really needed for every dataset?

**Describe the solution you'd like**
It is made optional.

**Describe alternatives you've considered**
Likely, no.
