New API for transforming inputs and outputs with encoders

Added

• Beta distribution loss for probabilistic models such as DeepAR (#160)

Changed

• BREAKING: Simplifying how to apply transforms (such as logit or log) before and after applying encoder. Some transformations are included by default but a tuple of a forward and reverse transform function can be passed for arbitrary transformations. This requires to use a transformation keyword in target normalizers instead of, e.g. log_scale (#185)

Fixed

• Incorrect target position if len(static_reals) > 0 leading to leakage (#184)
• Fixing predicting completely unseen series (#172)

Contributors

• jdb78
• JakeForsey