Major changes and bug fixes

Upgraded nGraph to v0.19.1
Capture Pass Checks: Capture Assign and other modifiers only if they operate on a variable and if the attribute validate_shape is true.
API to disable certain Tensorflow node types - useful for experiments and tests.
Added new Ops - RsqrtGrad, FusedMatMul, LogSoftMax
Added tf2ngraph.py - a script that transforms a TensorFlow graph to nGraph offline using Grappler.

Dependency information

TensorFlow v1.13.1
nGraph v0.19.1