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).

Some refactoring of the low-level pruning API

Added distiller/norms.py - for calculating norms of various sub-tensors.

ranked_structures_pruner.py:
-Removed l1_magnitude, l2_magnitude. Use instead distiller.norms.l1_norm
-Lots of refactoring
-replaced LpRankedStructureParameterPruner.ch_binary_map_to_mask with
distiller.thresholding.expand_binary_map
-FMReconstructionChannelPruner.rank_and_prune_channels used L2-norm
by default and now uses L1-norm (i.e.magnitude_fn=l2_magnitude was
replaced with magnitude_fn=distiller.norms.l1_norm)

thresholding.py:
-Delegated lots of the work to the new norms.py.
-Removed support for 4D (entire convolution layers) since that has not been
maintained for a longtime. This may break some old scripts that remove entire
layers.
-added expand_binary_map() explicitly so others can use it. Might need to
move to a different file
-removed threshold_policy()

utils.py:
-use distiller.norms.xxx for sparsity stats

* Fix bug: thresholding matrix cols should use dim=0 (issue #39)

See issue #39 for a description of the bug from @vinutah.

* thresholding test: fix device assignment