Remove single worker limitation in deterministic mode (#227)

Also:
* Single worker limitation not needed anymore, been fixed in PyTorch
  since v0.4.0 (https://github.com/pytorch/pytorch/pull/4640)
* compress_classifier.py: If run in evaluation mode (--eval), enable
  deterministic mode.
* Call utils.set_deterministic at data loaders creation if
  deterministic argument is set (don't assume user calls it outside)
* Disable CUDNN benchmark mode in utils.set_deterministic
  (https://pytorch.org/docs/stable/notes/randomness.html#cudnn)