[BUG] torch_forecasting_model depends on pytorch-lightining>=2.0

[eschibli]

Describe the bug
As of version 0.27 darts is not compatible with pytorch-lightning versions prior to 2.0 due to the dependency on the pytorch_lightning.callbacks.ProgressBar class. This class was introduced in pytorch-lightning 2.0. This isn't reflected in the requirements, which only specify pytorch-lightning>=1.5

To Reproduce
Attempt to import darts with pytorch-lightning<2.0. This will raise an error on line 43 of torch_forecasting_model.py, from pytorch_lightning.callbacks import ProgressBar.