Define QKV for recommender cross-attention

You are designing a deep-learning–based recommendation system that uses a Transformer-style cross-attention block to model the interaction between a user and a candidate item.

The model has these typical inputs:

• A user behavior sequence : a list of items the user has interacted with in the past, each already embedded as a vector (e.g., size d ).
• A candidate item whose relevance score you want to predict, also embedded as a vector of size d .
• Optional context features (time, device, location, etc.) that can also be embedded.

You decide to use a cross-attention layer somewhere in the model rather than only self-attention.

1. Propose a concrete way to define the Query (Q) , Key (K) , and Value (V) tensors in this cross-attention block using the inputs above. Explain what each of Q, K, and V represents semantically.
2. Give at least two different reasonable design choices for how to set up Q, K, and V (for example, one where the candidate item is the query and one where the user history is the query). For each design, explain:
   • What is used as Q, K, and V.
   • What interaction the attention mechanism is modeling.
   • Pros and cons or when that design is preferable.
3. Briefly explain how cross-attention here differs from self-attention within the user behavior sequence, and why cross-attention can be useful in recommendation systems.