Adding support for lag variables

Added

• Add "Release Notes" section to docs (#237)
• Enable usage of lag variables for any model (#252)

Changed

• Require PyTorch>=1.7 (#245)

Fixed

• Fix issue for multi-target forecasting when decoder length varies in single batch (#249)
• Enable longer subsequences for min_prediction_idx that were previously wrongfully excluded (#250)

Contributors

• jdb78