What is the purpose of using a key-value structure within the attention mechanism?

The purpose of using a key-value structure within the attention mechanism is to allow the model to attend to different aspects of each word in the input sequence based on its relationship to the current word being processed. In the attention mechanism, each word in the input sequence is transformed into three vectors: a query (q), a key (k), and a value (v). The query represents what the current word is "looking for", the key represents what each other word "offers", and the value represents the actual information content of each other word. The attention weights are calculated by comparing the query of the current word to the keys of all other words in the input sequence. This comparison determines how relevant each other word is to the current word. The key-value structure allows the model to decouple the process of determining relevance (using the keys) from the process of extracting information (using the values). This is important because the relevance of a word may not always be directly related to its information content. For example, in the sentence "The cat sat on the mat," the word "the" may not contain much information in itself, but it is still important for understanding the structure of the sentence. The key vector for "the" would capture its structural role, while its value vector would capture its actual information content (which is relatively low). By using a key-value structure, the attention mechanism can selectively attend to different aspects of each word, allowing it to capture more nuanced relationships and to generate more accurate and context-aware representations. Therefore, the key-value structure enhances the flexibility and expressiveness of the attention mechanism.