crossattention

Cross-attention is a mechanism used in transformer architectures to fuse information from two different sequences or modalities. In a cross-attention layer, the queries come from one source (for example, the current decoding context), while the keys and values come from another source (for example, the encoder output or a set of feature representations). This contrasts with self-attention, where queries, keys, and values all derive from the same sequence.

In multi-head cross-attention, each head computes its own Q from the first source and K,V from the second. The attention weights are computed as softmax(QK^T / sqrt(d_k)), and the output is the weighted sum of V, concatenated across heads and projected back to the model dimension. This allows the model to attend to different aspects of the second sequence while processing the first.

Common contexts for cross-attention include encoder-decoder models such as neural machine translation, where the decoder attends to encoder representations to generate each target token. It is also central to multimodal transformers that align text with images, audio, or other modalities, enabling conditional generation or cross-modal reasoning.

Challenges and considerations include computational complexity, which is quadratic in the lengths of the interacting sequences, and potential alignment ambiguity between the sources. Variants and optimizations, such as limiting attention to specific segments or applying efficient attention mechanisms, are areas of ongoing research.