RFC: add isin for elementwise set inclusion test

[lucascolley]

Prior art

• NumPy - https://numpy.org/doc/stable/reference/generated/numpy.isin.html
• CuPy - https://docs.cupy.dev/en/latest/reference/generated/cupy.isin.html
• Dask - https://docs.dask.org/en/stable/generated/dask.array.isin.html
• JAX - https://jax.readthedocs.io/en/latest/_autosummary/jax.numpy.isin.html
• PyTorch - https://pytorch.org/docs/stable/generated/torch.isin.html
• ndonnx - https://ndonnx.readthedocs.io/en/latest/api/ndonnx.additional.html#ndonnx.additional.isin
• MLX - isin is not present yet

Motivation

This function is used in scikit-learn. They've implemented it in terms of the standard, and that implementation could find a home in array-api-extra: data-apis/array-api-extra#34. @asmeurer suggested there that we should also consider adding this to the standard.

[asmeurer]

Another potential reason for adding it is that it uses a nontrivial implementation which depends on some heuristics based on the input size.

[rgommers]

Thanks @lucascolley. I've added ndonnx (which has it) and MLX (which doesn't) to the issue description.

This seems like a very reasonable proposal to me. Implementing isin in terms of other primitives in the standard is a little complex indeed.

The return type should always be a boolean array. The NumPy docs say it can be a bool for a single input element, but that's actually a bug in the docs. I checked NumPy, JAX, and PyTorch and all return a 0-D array.