small tensor masking API refactoring (#499)

Added masking primitives:
 -mask_tensor
 -create_mask_threshold_criterion
 -create_mask_level_criterion
 -create_mask_sensitivity_criterion

 These APIs have a clearer name and communicate their
 responsibility better: create a tensor mask, based on
 some criterion.  Previously,
 distiller.pruning.create_mask_threshold_criterion was
 named distiller.threshold_mask which did not communicate
 well what this function did.
 Masking functionality is no longer hidden
 inside the Pruner instances, so they can be used directly
 by an application, or to compose new Pruner classes.

Removed file distiller.pruning.pruner:
 -The base-class _ParameterPruner is useless and adds
 needless details to the implementation.

AGP: Separated the pruning-rate schedule from the
 rest of the logic.  This allows us to mix-and-match different
 pruning-rate schedules (just like LR schedulers).