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