Check if correct version of PyTorch is installed.

The 'master' branch now uses PyTorch 0.4, which has API changes that
are not backward compatible with PyTorch 0.3.

After we've upgraded Distiller's internal implementation to be
compatible with PyTorch 0.4, we've added a check that you are using
the correct PyTorch version.

Note that we only perform this check in the sample image classifier
compression application.