RevIn for GluonTS

## Description
I have a python implementation of the Reversible Instance Norm and i would like to add it to GluonTS. Where is the correct dict, to add this implementation?

## References
[Original GitHub Implementation](https://github.com/ts-kim/RevIN)
[Paper Link](https://openreview.net/pdf?id=cGDAkQo1C0p)