You will be shown an excerpt from a computer science scientific research paper, followed by a question about the paper. Please answer the question. Do not include any text in your response other than the answer.

Context: This model takes inspiration from the tradeoff between the soft and hard attentional models proposed by ?) to tackle the image captiongeneration task. In their work, soft attention refers to the global attentionapproach in which weights are placed “softly” over all patches in the sourceimage. The hard attention, on the other hand, selects one patchof the image to attend to at a time. While less expensive at inference time, thehard attention model is non-differentiable and requires more complicatedtechniques such as variance reduction or reinforcement learning to train.

Question: How are hard attention models different from soft attention models?
Soft attention model's weights are placed "softly" over all patches in the source image. while Hard attention models selects one patch of the image to attend at a time. it's also, none-differentiable, requires more complicated techniques and less expensive at inference time.