Synchronize with submitit Slurmexecutor python arg addition

[berombau]

Motivation

Synchronizing with now merged submitit change (facebookincubator/submitit#1729) that allows to pass in a "python" variable to SlurmExecutor.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

This minimal added optional variable works for my setup.

Related Issues and PRs

The previous PR #2654 does the same, but the PR was closed before it could be merged.

[adamtupper]

This would be an amazing addition. I cannot use the Submitit plugin without it on our cluster.

[lauritsf]

This contribution is of high value to people working on larger HPC clusters, where the preferred approach is to use singularity containers.

The naming and default match the existing init keys.

[omry]

Thanks, I will fix this.

[omry]

Thanks for the PR. This looks like a useful addition, but it needs a bit more before I can take it:

• Add tests covering the new hydra.launcher.python / slurm_python behavior.
• Add a plugin news fragment for hydra-submitit-launcher.

Please also make sure the implementation handles the Submitit version requirement cleanly, since Hydra currently allows older Submitit versions that may not support this constructor argument.