Add SegRNN Implementation

[lss-1138]

What does this PR do?

I have added an implementation of the SegRNN model along with testing examples. SegRNN is an innovative RNN-based model designed for Long-Term Time Series Forecasting (LTSF). It incorporates two fundamental strategies:

1. Replacing point-wise iterations with segment-wise iterations.
2. Substituting Recurrent Multi-step Forecasting (RMF) with Parallel Multi-step Forecasting (PMF).

I have made appropriate adjustments to the original SegRNN to make it suitable for imputation tasks.

Before submitting

• This PR is made to fix a typo or improve the docs (you can dismiss the other checks if this is the case).
• Was this discussed/approved via a GitHub issue? Please add a link to it if that's the case.
• I have commented my code, particularly in hard-to-understand areas.
• I have written necessary tests and already run them locally.