Update create_op.md
Update create_op.md

- Use TensorFlow 2.0 style (eager execution) in python example - remove `tf.Session`
- Fix the GPU Kernel code, including missing headers, adding the operation registration, and fixing a wrong include ("example.h" doesn't exist)
- Add a section on how to compile custom ops with CUDA kernels.
tensorflow/docs,tensorflow/docs,tensorflow/docs