Add iTransformer (Inverted Transformer) to DNN models

[ivelin]

Is your feature request related to a current problem? Please describe. It appears that DNN models are advancing on a monthly basis. The iTransofmer paper from Dec 2023 claims to be a more resource efficient model with better accuracy than TiDE, PatchTST, TimesNet and other recent DNNs for time series.

Describe proposed solution Consider adding iTransformer to the darts DNN model set.

Additional context iTransformer paper: https://arxiv.org/pdf/2310.06625.pdf

iTransformer implementation: https://github.com/thuml/iTransformer

[lamkhatinh]

+1

[strakehyr]

I wonder what performance we'd get to pairing this up with a symmetric layer of reversible instance normalization...