Migrate from libtensorflow to TFRT

[lantiga]

https://blog.tensorflow.org/2020/04/tfrt-new-tensorflow-runtime.html

[DvirDukhan]

@lantiga still relevant?
From their readme:
Note: TFRT is an early stage project and is not yet ready for general use.
also, how it will affect #565?

[lantiga]

Hi @DvirDukhan , I would close this. TFRT looks far away enough in the future.