Silence layer deletes data during backprop

[cvondrick]

During Backward_* in the Slience layer, it performs the following operation when propagate_down[i] is set to true:

caffe_set(bottom[i]->count(), Dtype(0), bottom[i]->mutable_cpu_data());

and similarly for the GPU.

Usually this code will not run because the Silence layer does not generally need backprop. However, if you force the network to do backprop, this will have the consequence of overwriting the bottom data blob with all zeros.

I think the correct behavior is to set the bottom diff to 0:

caffe_set(bottom[i]->count(), Dtype(0), bottom[i]->mutable_cpu_diff());

Should this be changed?

Thanks!

[jeffdonahue]

Thanks for reporting @cvondrick. You're right, this is a bug and should be fixed. In practice though I think propagate_down should always be set to false since SilenceLayer won't be on a path to a loss (unless manually overridden with propagate_down: true in the LayerParameter, which would be odd), so this probably isn't affecting anyone...

[cvondrick]

I agree that is an odd case, but I think it can also be triggered by setting the entire network to do force_backward (which is how I found this bug).

[jeffdonahue]

You're right, that would trigger it as well. I'll send a PR with the fix in a second. Thanks again!