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: v0.9.8...v0.9.9