Gluonts

See full diff here: https://github.com/awslabs/gluonts/compare/v0.13.0...v0.14.0

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)