Add version header to CLI config files

[carmocca]

What does this PR do?

Uses: omni-us/jsonargparse#79

Example config:

# pytorch_lightning==1.7.0dev
seed_everything: null
trainer:
  logger: true
  checkpoint_callback: null
  enable_checkpointing: true
  callbacks: null
  default_root_dir: null
  gradient_clip_val: null
  gradient_clip_algorithm: null

Does your PR introduce any breaking changes? If yes, please list them.

The jsonargparse version was updated.

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cc @Borda @carmocca @mauvilsa

[krshrimali]

🚀