[ENH] Implementing TimeXer model

[PranavBhatP]

Is your feature request related to a problem? Please describe.
Implement TimeXer model based on the implementation in https://github.com/thuml/TimeXer and outlined in
https://arxiv.org/abs/2402.19072

[PranavBhatP]

I will be trying and implementing this model (as an experiment initially) with the design proposed under sktime/enhancement-proposals#39 as well as the existing design to compare these two. Considering that the model is also from thuml library, I feel that it is a right step in ensuring that the PTF 2.0 design makes it easier to integrate thuml models, if required in the future.