Make the exponential decay lr schedule available

[twuebi]

Right now, the exponential decay lr schedule is not available for sticker trainand sticker pretrain. Once #145 is merged, it would make sense to have it available for both subcommands.

This may clutter the command line arguments a bit since we then have:

• Plateau decay

  • lr_scale
  • lr_patience

• Exponential decay

  • decay_rate
  • decay_exponent

Maybe it would make sense to move the learning-rate schedule related things to the config file.