[#57] Ray integration prototyping

[jessezbj]

This is a draft pull request to show how Ray works with autodist. Not ready to merge yet.

[jessezbj]

to run autodist on ray RAY_HEAD_ADDRESS=<ip:6379> python <full_path>/casl/autodist/examples/linear_regression_ray.py

tested with https://s3-us-west-2.amazonaws.com/ray-wheels/latest/ray-1.2.0.dev0-cp36-cp36m-manylinux2014_x86_64.whl and tensorflow 2.2.0 on 3 nodes with 6 GPUs.

[sonicxml]

this is super cool