Gluonts

Backporting:
- Fix Rotbaum serialization and deserialization 3068 by pantanurag555
- Fix Rotbaum to handle short series 3073 by leica2023

Backporting fixes:
- Fix edge cases in metric computation 3037 (lostella)
- Refactor tests for ev.aggregations 3038 (lostella)
- Rotbaum: Add item-id to forecast. 3049 (jaheba)
- Fix mypy checks 3052 (lostella)

Fixing issues with updated requirements, which would prevent the package from installing alongside old versions of PyTorch Lightning:
- Fix lightning import (3018 by shchur)
- Update requirements-pytorch.txt (3019 by shchur)

Backporting fixes:
- [torch] Return a model even if callback has no best model path 2952 by kashif
- Move from pytorch_lightning to lightning 3013 by shchur

Backporting fixes:
- Raise warning in QuantileForecast.mean when mean is not there 2843 by lostella
- [R] Bug fix for running R methods via ParallelizedPredictor 2983 by rshyamsundar
- Fix Forecast plot method 3006 by lostella

Backporting fixes:
- Turn type comparison into isinstance 2958 by lostella
- Fix Cython version in XGBoost tests 2966 by lostella
- Clean up RepresentablePredictor 2967 by lostella
- Fix: use isinstance instead of type comparison 2973 by melopeo
- Fix ArrowDecoder.decode to return instead of yield 2976 by lostella
- Unpin pyarrow version 2977 by lostella