If you have a single embedded vector, it should be clear that the self-attention matrix cannot be an arbitrary NxN but only a 1x1, follow the fix I have added t

If you have a single embedded vector, it should be clear that the self-attention matrix cannot be an arbitrary NxN but only a 1x1, follow the fix I have added to my previous comment. Also if K, Q, V and the self-attention matrix have the same dimensions even when the embed_d and sequence dim have different sizes there is something wrong.