Fix benchmarking scripts

[devernay]

• closes https://github.com/nerfstudio-project/nerfstudio/issues/1000
• see also https://github.com/nerfstudio-project/nerfstudio/issues/806
• launch_train_blender.sh:
  • add -s option to launch a single job per GPU
  • add -v option to use tensorboard instead of wandb
  • set nerfacto options according to https://github.com/nerfstudio-project/nerfstudio/issues/806#issuecomment-1284327844
  • use a single timestamp for all training jobs
  • last GPU was ignored
  • kill all subprocesses when script is terminated
  • print the eval script command-line
• launch_eval_blender.sh:
  • add shebang
  • add -s option to launch a single job per GPU
  • last GPU was ignored
  • kill all subprocesses when script is terminated
• update benchmarking doc