Attention correction

Fixing attention. Added a basic test value.
That's because when the attention mechanism is applied repeatedly by the AttentionTransition, weights produced at every step are concatenated, and I did not want them to a weird shape (input_time_step, attended_time_step, number_in_the_batch). But you're right, people might expect `weights` to have the shape as `mask`. So we should think and take a decision.