[ENH] implementing `iTransformer` from `tslib`.

[PranavBhatP]

Is your feature request related to a problem? Please describe.
Implementing the iTransformer model in PTFv2.

Basis: https://github.com/thuml/iTransformer/blob/main/model/iTransformer.py
Paper: https://arxiv.org/pdf/2310.06625