Activation Histograms (#254)

Added a collector for activation histograms (sub-class of
ActivationStatsCollector). It is stats-based, meaning it requires
pre-computed min/max stats per tensor. This is done in order to prevent
the need to save all of the activation tensors throughout the run.
The stats are expected in the format generated by
QuantCalibrationStatsCollector.

Details:

* Implemented ActivationHistogramsCollector
* Added Jupyter notebook showcasing activation histograms
* Implemented helper function that performs the stats collection pass
  and histograms pass in one go
* Also added separate helper function just for quantization stats
  collection
* Integrated in image classification sample
* data_loaders.py: Added option to have a fixed subset throughout
  within the same session. Using it to keep the same subset between
  the stats collection and histograms collection phases.
* Other changes:
  * Calling assign_layer_fq_names in base-class of collectors. We do
    this since the collectors, as implemented so far, assume this is
    done. So makes sense to just do it in the base class instead of
    expecting the user to do it.
  * Enforcing a non-parallel model for quantization stats and
    histograms collectors
  * Jupyter notebooks - add utility function to enable loggers in
    notebooks. This allows us to see any logging done by Distiller
    APIs called from notebooks.