LoginNodes should fall back to HeadNode's SSH key when no Ssh section is provided

LoginNodes should fall back to HeadNode's SSH key when no Ssh section is provided: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). I'm setting up a Slurm cluster with both a HeadNode and a LoginNodes pool. I want to access everything (head node and login nodes)…