CovarianceLayer Backward incorrect

Thanks for the port of your code. (I've learned immensely from it.)

FYI, the implementation of CovarianceLayer<>::Backward_cpu() is incorrect.
The first thing it does is get a mutable pointer to the top Blob. During backprop it shouldn't be modifying the top Blob, only the bottom Blob.

```
template
void CovarianceLayer::Backward_cpu(const vector*>& top,
const vector& propagate_down,
const vector*>& bottom) {
Dtype* top_diff = top[0]->mutable_cpu_diff();
...
```

cheers,
Doug