Fix slurm multinode example

[ffrancesco94]

What does this PR do?

The SLURM multinode submit script doesn't work for multiple reasons (see also #1239 which is still open). This PR aims at solving some of those issues, namely:

• Typo in the $CMD command
• $SLURM_NNODES was recently deprecated in favour of $SLURM_JOB_NUM_NODES
• If multinode setup involves multiple GPUs, it has to be enforced with --multi-gpu and the rank of each process has to be set with --machine-rank
• If multiple GPUs are present, the complete_nlp_example doesn't handle distributed evaluation correctly. In particular, either the evaluate GLUE mrpc metric has to be loaded with the --num_process and --process_id flags (which will subsequently fail due to several breakages due to recent datasets  evaluate#542 & related) or metric computation has to happen only on the main process. This PR goes for the second.

Fixes #3206

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings. NA
• Did you write any new necessary tests? NA

Who can review?

@muellerzr or @SunMarc