RFC: add APIs for getting elements via a list of indices (i.e., `take`, `take_along_axis`, etc)

先从提案以及 sorting_functions.md 和 set_functions.md 中链接的规范部分开始，然后比较其中引用的 NumPy、CuPy、Dask、MXNet、Torch 和 TensorFlow API。当项目就 take/put 系列的范围和语义达成一致，并且相关的 Array API 规范得到更新时，这项工作就完成了。

Proposal

Add APIs for getting and setting elements via a list of indices.

Motivation

Currently, the array API specification does not provide a direct means of extracting and setting a list of elements along an axis. Such operations are relatively common in NumPy usage either via "fancy indexing" or via explicit take and put APIs.

Two main arguments come to mind for supporting at least basic take and put APIs:

1. Indexing does not currently support providing a list of indices to index into an array. The principal reason for not supporting fancy indexing stems from dynamic shapes and compatibility with accelerator libraries. However, use of fancy indexing is relatively common in NumPy and similar libraries where dynamically extracting rows/cols/values is possible and can be readily implemented.

2. Currently, the output of a subset of APIs currently included in the standard cannot be readily consumed without manual workarounds if a specification-conforming library implemented only the APIs in the standard. For example,

   • argsort returns an array of indices. In NumPy, the output of this function can be consumed by put_along_axis and take_along_axis.
   • unique can return an array of indices if return_index is True.

Background

The following table summarizes library implementations of such APIs:

While most libraries implement some form of take, fewer implement other complementary APIs.