Gluonts

Backporting fixes:
* Fix r_forecast wrapper to shift start date when truncating time series (2216 by abdulfatir)
* Fix dtype bug in piecewise_linear and add a test (2224 by rshyamsundar)
* Fix bug in to_quantile_forecast (2225 by eugeneteoh)
* Fix gluonts.mx.trainer.Trainer in case of empty data loader (2228 by lostella)
* Fix feed-forward models when features are provided (2238 by lostella)


**Full Changelog**: https://github.com/awslabs/gluon-ts/compare/v0.9.8...v0.9.9

Backporting fixes:
- Fix `SymbolBlock` serde issue (2187 by lostella)

Backporting fixes:
- Fix dtype for "item_id" column in metrics dataframe (https://github.com/awslabs/gluon-ts/pull/2183 by lostella)
- Fix recursive case for gluonts.mx.batchify.stack (https://github.com/awslabs/gluon-ts/pull/2184 by lostella)
- Fix item_id values in ConstantValuePredictor (https://github.com/awslabs/gluon-ts/pull/2192 by codingWhale13)

Backporting fixes:
- Fix: DistributionLoss not encodable (2092 by jaheba)

* Re-add support for Python 3.6 in v0.9.x. (2032 by jaheba)

Backporting fixes:

* Fix: use of `num_parallel_samples` in deepAR (1968 by kashif)
* Fix: torch DeepAR observed indicator in multivariate case (1969 by kashif)
* Fix `OffsetSplitter` for negative offsets (1986 by lostella)
* Fix mean in `AffineTransformedDistribution` (2015 by stailx)

Backporting fixes:
* Fix: Hard threshold positive distribution parameters (1950 by lostella)
* Fix forecast keys (quantiles) output by TemporalFusionTransformer (1952 by lostella)