Gluonts

Backporting fixes:
- Faster index building in PandasDataset 2663 by huibinshen
- Speed up PandasDataset.from_long_dataframe 2665 by lostella
- Fix DateSplitter when split date is before start 2670 by gorold
- Remove creation of ragged sequences in MultivariateGrouper 2671 by abdulfatir

Backporting fixes:
- Fix PyTorch training loop 2643
- Fix norm-freq to consider freq starts. 2645
- Fix call to extractall 2648

Backporting fixes:
- Fix: avoid automatic device detection via serialized tensors when deserializing PyTorchPredictor. 2576 by shubhamkapoor
- Fix Map representation. 2579 by jaheba
- serde: Fix encoding of dtypes. 2586 by jaheba
- Add assertion to split function ensuring valid windows 2587 by MarcelK1102
- Ensure dtype on feat_time in torch DeepAR. 2596 by jaheba
- Expose aggregation method in ensemble NBEATS, fix forecast shape 2598 by lostella
- Fix requirements following breaking change in setuptools 2604 by lostella
- Add gluonts.util.safe_extract 2606 by lostella

Backporting fixes:
- Update workflow actions to latest versions 2447 by jaheba
- Simplify setup.py 2525 by jaheba
- Fix dataclass eventual handling. 2530 by jaheba
- Improve README example 2538 by lostella
- Change SeasonalNaive fallback predictor to nanmean 2549 by abdulfatir
- Compact PandasDataset string representation 2558 by lostella
- Update MANIFEST.in. 2566 by jaheba
- Add NaN validation to Evaluator 2568 by abdulfatir

Backporting fixes:
- Make serde.dataclass always kw-only. (2428 by jaheba)
- Fix `serde.dataclass` inheritance handling. (2512 by jaheba)
- Fix `QuantileForecast.quantile` in case only mean is stored (2513 by lostella)
- Remove mypy plugin for dataclass. (2514 by jaheba)
- GH Actions: Use authenticated requests for just. (2522 by jaheba)
- Fix `aggregate_valid` for non-numerical columns (2526 by lostella)

Backporting fixes:
- itertools.select. 2426 by jaheba
- Fix dataclass handling of member inheritance. 2492 by jaheba
- Fix: sort dataset keys in error message when importing non-existing dataset 2497 by lostella
- Fix DateSplitter for multiples of base frequencies 2500 by lostella
- Fix docstrings according to docformatter 2501 by lostella
- Add examples to docstring for periods_between 2504 by lostella
- Cap numpy compatibility in mxnet extra requirements 2506 by lostella
- Pin docformatter version. 2507 by jaheba
- Docs: Fix install instructions. 2508 by jaheba