Filter removal: support filter removal for VGG

This is a temporary implementation that allows filter-removal and
netowrk thinning for VGG.
The implementation continues the present design for network thinning,
which is problematic because parts of the solution are specific to
each model.

Leveraging some new features in PyTorch 0.4, we are now able to provide a
more generic solution to thinning, which we will push to 'master' soon.
This commit bridges the feature gap, for VGG filter-removal, for the
meantime.