Enable compute (training/inference) on the CPU

In compress_classifier.py we added a new application argument: --cpu
which you can use to force compute (training/inference) to run on the CPU 
when you invoke compress_classifier.py on a machine which has Nvidia GPUs.

If your machine lacks Nvidia GPUs, then the compute will now run on the CPU
(and you do not need the new flag).

Caveat: we did not fully test the CPU support for the code in the Jupyter 
notebooks.  If you find a bug, we apologize and appreciate your feedback.