Created by: baojun-nervana

This is update ngraph to handle multiple threading safe; prepare to enable parallel executor

• Removed a few static class members which can cause issues for parallelization
• changed backed_ to be per instance
  • used a weak pointer to hold the backend to keep it alive in desired lifetime; otherwise it may run into a case it will be destroyed too early
  • a mutex lock was used to keep it thread safe - this is subject to optimization which we are looking at.

CC. @pawelpiotrowicz @LeoZhao-Intel