Set up environment variables for interactive hosted API service

Created by: dgrnbrg-meta

When you try to run the API service on a slurm node, but you're not inheriting the slurm environment, you get a very strange error:

... snip ...
  File "/shared/home/dgrnbrg/metaseq/metaseq/checkpoint_utils.py", line 316, in _is_checkpoint_sharded
    size_ratio = max(sizes) / min(sizes)
ValueError: max() arg is an empty sequence

It turns out this is due to incorrectly inferred config in metaseq/distributed/utils.py.

Patch Description This adds a warning & a sane default (use the entire node that the API server is being run on).

Testing steps I tried removing each of these env vars, and they're all necessary: any subset causes different crashes.

I ensured I could run & query the API successfully, using the fully sharded 175b param model.