Expose gigl vars as env vars for the custom luancher

[kmontemayor2-sc]

Do this so custom launchers can get the task config, etc.

The alternative approach here is that we can use omegaconf but as we discussed offline that may be more complicated as the resolution would change often.

[svij-sc]

Might be good opportunity to also consolidate existing VAI launchers to use env vars too instead of injected runtime args:

GiGL/gigl/src/common/vertex_ai_launcher.py

Lines 256 to 264 in 454b54b

	job_args = (
	[
	f"--job_name={job_name}",
	f"--task_config_uri={task_config_uri}",
	f"--resource_config_uri={resource_config_uri}",
	]
	+ (["--use_cuda"] if use_cuda else [])
	+ ([f"--{k}={v}" for k, v in args.items()])
	)

Adopting one distinct pattern will make it easier for us to maintain the different code paths, and I am happy to adapt the env variables given prior discussed arguments as the golden path, but lets also old our old injection paths in existing launchers.

[svij-sc]

I'd prefer to have this PR in first as the other changes may be somewhat complicated still (and I'm not sure we need all of these things to be exposed globally)

Approving conditionally - only for the purposes of unblocking. I am hoping we do a fast follow up here to address concerns outlined below:

1. https://github.com/Snapchat/GiGL/pull/642/changes#r3268620564
2. https://github.com/Snapchat/GiGL/pull/642/changes#r3269575120