Skip GPU selection prompt if only 1 possible GPU

[philipbutler]

Description

Skip GPU selection prompt if only 1 possible GPU

Before:
(1 GPU available for leaderboard)

After:
(1 GPU available for leaderboard)

(multiple GPUs available for leaderboard)

Checklist

Before submitting this PR, ensure the following steps have been completed:

• ❌ (skipped, don't have runners set up) Run the slash command /verifyruns on your own server.
  • Run the cluster bot on your server:

    python discord-bot.py

  • Start training runs with the slash command /verifyruns.
  • Verify that the bot eventually responds with:

    ✅ All runs completed successfully!
    

    (It may take a few minutes for all runs to finish. In particular, the GitHub
    runs may take a little longer. The Modal run is typically quick.)
    For more information on running a cluster bot on your own server, see
    README.md.