Add data_loading time measurement to benchmarking   (#1079)
Add data_loading time measurement to benchmarking   (#1079)

* Measure data_loading time and add it to whole iteration time

* move data_loading value to fprop key

* Switch to optional 'data_loading' dict key; add support for GAN networks

NervanaSystems/neon,NervanaSystems/neon,NervanaSystems/neon,NervanaSystems/neon