Completions inconsistency

Every object gets their own handler accessible from the `OpenAI` class except for completions (both completions and chat completions). They each should get their own handler. This will also help the API match up better with the official APIs provided by OpenAI

首先阅读 OpenAI 类，并检查现有对象处理器是如何暴露的。将当前对 completions 和 chat-completions 的访问模式与官方 API 的结构进行比较；完成的标准是每个对象都有自己的处理器，并且公共 API 与该结构保持一致。

由索引模型根据 Issue 内容生成。