feat(rollout)!: Rollout Schedulers

[HCookie]

Closes ecmwf/anemoi-core#14

Rollout Schedulers

Expand the ways to describe rollout, and provide an interface to schedule updates

New default rollout config

# length of the "rollout" window (see Keisler's paper)
rollout:
  _target_: anemoi.training.schedulers.stepped.EpochStepped
  minimum: 1
  maximum: 12
  # increase rollout every n epochs
  every_n_epochs: 1
  # Control the incrementing of the rollout window
  increment:
    step:
      0: 0
      200000: 1 # After 200k steps, increment by 1 every 1 epoch

Can step by epoch, step, and control the increment based on either the step or epoch.

Additonally, formally add random steppers.

Todo

• Integrate with the data loader
• Ensure that the randomness is seeded appropriately and broadcast
• Ensure restartability and ability to change config