example selectors

示例选择器的作用是选择正确的少量示例给到提示词

custom example selector

1. 继承 BaseExampleSelector，并实现 select_examples 抽象方法

example selector type

name	desc
Similarity	Uses semantic similarity between inputs and examples to decide which examples to choose.
MMR	Uses Max Marginal Relevance between inputs and examples to decide which examples to choose.
Length	Selects examples based on how many can fit within a certain length
Ngram	Uses ngram overlap between inputs and examples to decide which examples to choose.

直接使用 langchain 内置类即可，详见: https://python.langchain.com/docs/how_to/#example-selectors