Deprecate trainer.slurm_job_id

[ananthsub]

Proposed refactor

Deprecate the trainer property here: https://github.com/PyTorchLightning/pytorch-lightning/blob/6b728713bb3b35ad58cd0085acaa443b33ab03ac/pytorch_lightning/trainer/trainer.py#L1731-L1744

Motivation

This property doesn't depend on the Trainer at all: this property can be converted into a utility function/staticmethod instead.
Proposal: Migrate this logic into the SLURMEnvironment class as a staticmethod. Doing this will consolidate SLURM environment variable code in Lightning and simplify the Trainer. https://github.com/PyTorchLightning/pytorch-lightning/search?q=slurm_job_id

Pitch

1. Move this logic to the SLURMEnvironment class as a staticmethod

@staticmethod
def job_id() -> Optional[int]:

@staticmethod
def job_name() -> Optional[str]:

2. Then replace usages of trainer.slurm_job_id with SLURMEnvironment.slurm_job_id() / slurm_job_name
3. Then deprecate trainer.slurm_job_id

Additional context

Part of #7740

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cc @justusschock @awaelchli @akihironitta @tchaton @kaushikb11 @Borda

[awaelchli]

Yes agree! Probably can drop the slurm prefix

@staticmethod
def job_id() -> Optional[int]:

[AndresAlgaba]

As it is marked as a "good first issue", do you mind if I create a PR for this issue? I will have time next Monday and Tuesday to work on this :).

[ananthsub]

@AndresAlgaba that'd be great!