Hotfix for accuracy interfering with training

[Noiredd]

Instead of reusing "unused" memory of the bottom blob (diff), it's safer to allocate some internal blob (gpu_buffer_) to make sure the layer does not interact with external memory that might be potentially used elsewhere (as was demonstrated in #5981). Bug was initially introduced in 62e0c85.

In the long run, we might want to optimize this to eg. only allocate GPU memory (instead of an entire blob), or even take a closer look at what is exactly going on in that buffer (i.e. how much memory do we actually need). On the other hand, using Accuracy layer during training is not very common usage - so we might leave this PR pending for now (for the affected users to pull) and only accept after said optimizations.

[Noiredd]

@shaibagon Can we optimize this memory-wise in any way? I.e. do we need the intermediate blob to be as large as the whole bottom[0]? If not, I say we merge. @shelhamer?

[ghost]

Hi,
what the progress on merging this fix? It is already almost a month for caffe not being able to calculate/display accuracy statistics in training phase.

[shelhamer]

@Noiredd see #6202 for a combined fix to the scratch usage of bottoms diffs. I decided to clear the diffs in that fix instead of needing separate memory. Please let me know what you think of that choice.