Have documentation to point to all our environment variables and their meaning

[miladm]

📚 Documentation

Prepare a documentation to point to all our environment variables and their meaning. This world should be a forcing function to (1) make the yaml file up to date (2) rename it to something like env_vraiable_definitions.yaml, (3) start a workstream to trim down on these env variables to avoid usability pain.

https://github.com/pytorch/xla/blob/master/configuration.yaml

@tengyifei @yaoshiang for viz and support

[miladm]

Perhaps we can add a field that explains which env var is user impacting which one is not (i.e. internal to the function of torchxla stack).