[RFC]: DLPack C Function for Speed Exchange

首先阅读链接的 DLPack issue 175 以及当前的 DLPack 交换上下文。定义提议的字段 mypackage.Tensor.__dlpack_c_exchange_api__ 应如何针对数组库进行标准化，包括它与现有 Python 交换机制及回退行为之间的关系；当 RFC 形成了经一致同意的规范时，这项工作即告完成。

This is a cross ref RFC on DLPack based exchange. As of now, DLPack exchange relies on python functions such as tensor.__dlpack__(). While they works well for common cases, the general overhead of such exchange is at the level of 0.2-0.3 us for very well optimized version, and can go up to 0.4-1 us for less optimized implementation.

For a function that takes three arguments f(a, b, c), assume we run DLPack exchange for each argument, the general conversion overhead usually gets to around 0.7us - 3us.

While such overhead can be acceptable in many settings, in GPU applications the extra 1-3us overhead can still be significant. For a kernel that takes 2us to finish, 0.7 us means 30% additional overhead in execution

Recently, we propose to develop a set of specific C functions to help DLPack based exchange for array libraries that works on C extensions, please see more context here

https://github.com/dmlc/dlpack/issues/175

In the context of array-api, it would be useful to help standardize the specific field for such speed exchange

• mypackage.Tensor.__dlpack_c_exchange_api__

Note that the proposed speed exchange function can be used in conjunction with the current DLPack exchange, to gracefully handle fallback cases.