[tune] TF2.0 TensorBoard support (#5547)
[tune] TF2.0 TensorBoard support (#5547)

* Fix tensorboard log issue with tensorflow2.0

* tf2 support

richardliaw/ray,pcmoritz/ray-1,robertnishihara/ray,robertnishihara/ray,richardliaw/ray,pcmoritz/ray-1,stephanie-wang/ray,pcmoritz/ray-1,ray-project/ray,richardliaw/ray,ray-project/ray,robertnishihara/ray,ray-project/ray,ujvl/ray-ng,ray-project/ray,robertnishihara/ray,richardliaw/ray,pcmoritz/ray-1,stephanie-wang/ray,ray-project/ray,ray-project/ray,ujvl/ray-ng,robertnishihara/ray,richardliaw/ray,ujvl/ray-ng,richardliaw/ray,stephanie-wang/ray,richardliaw/ray,pcmoritz/ray-1,richardliaw/ray,robertnishihara/ray,pcmoritz/ray-1,ray-project/ray,robertnishihara/ray,stephanie-wang/ray,ujvl/ray-ng,stephanie-wang/ray,robertnishihara/ray,stephanie-wang/ray,pcmoritz/ray-1,ray-project/ray,stephanie-wang/ray,stephanie-wang/ray,stephanie-wang/ray,pcmoritz/ray-1,robertnishihara/ray