Document environment variables

[steventk-g]

Created a yaml file that captures all relevant environment variables with description, type, and default value.

Variables are grouped into the following:

• Build variables
• Debug variables
• Device variables
• PJRT variables
• XRT variables
• Feature variables

[JackCaoG]

lol I felt like we should not make this configurable

[steventk-g]

I wonder if we should include it here for the sake of documentation, and remark that it shouldn't be changed. My intention in including it was just to cover every environment variable that shows up in this repo.

xla/torch_xla/__init__.py

Line 66 in f858155

_set_missing_env('TPU_ML_PLATFORM', 'PyTorch/XLA')

[JackCaoG]

Mostly LGTM. Thanks for putting this together! Is it possible to group similar env var for better reading expericence?

For example we can split them into

1. XRT related
2. Build related
3. Debug related
4. Feature enablement
5. Device configuration
6. Distributed related
7. ...

[will-cromar]

Wow! Thanks for laying all of this out. LGTM at least for the variables I know about

I agree with Jack's comment that it would be clearer if you categorized the variables, especially separating the build-time variables from the run-time variables and separating the XRT configuration from the rest.

@JackCaoG Do we also want some designation of which variables are stable and which might get removed in the future? Or are we committed to actually supporting all of these?

[JackCaoG]

yea I think in this or I am fine with only group them by cateratory. We can update it based on stable/to_be_removed in a follow up pr.

[JackCaoG]

Thanks!