Introducing TensortRT Operator to TF which can run (sub)graphs in

highly optimized TensorRT engines.  This commit is a merged version of
many commits by

   benbarsdell    <bbarsdell at nvidia.com>
   deadeyegoodwin <davidg at nvidia.com
   jjsjann123     <jiej at nvidia.com>
   samikama      <skama at  nvidia.com>