* Initial multi-precision training

Adds fp16 support via apex.amp
Also switches communication to apex.DistributedDataParallel

* Add Apex install to dockerfile

* Fixes from @fmassa review

Added support to tools/test_net.py
SOLVER.MIXED_PRECISION -> DTYPE \in {float32, float16}
apex.amp not installed now raises ImportError

* Remove extraneous apex DDP import

* Move to new amp API