Add slurm_template parameter to SlurmExecutor (and use it in SlurmBatchRequest)

[Kipok]

We have a need to customize the current slurm template used here https://github.com/NVIDIA/NeMo-Run/blob/main/src/nemo_run/core/execution/templates/slurm.sh.j2. I imagine there might be more use-cases where customization is necessary, so it might be a good idea to accept custom template as a parameter (with existing one as default)