[ENH] Implement s2s Linear Forecaster models family

[rwtarpit]

Reference Issues/PRs

Implements LinearForecaster family of models (linear, nlinear and dlinear) following the PR :
#3146

What does this implement/fix? Explain your changes.

This PR implements a simple 1 layered MLP based Linear Forecaster as a baseline for series2series deep learning forecasters.

Does your contribution introduce a new dependency? If yes, which one?

No

Any other comments?

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[hadifawaz1999]

I took a first look, a small comment to begin,i will check the rest

[hadifawaz1999]

LGTM ! very nice, @MatthewMiddlehurst @TonyBagnall @alexbanwell1 let me know if you have any comments before merging

[alexbanwell1]

Looks good to me. Can't comment on the actual model but the forecasting module changes and series to series mixin look sensible

[MatthewMiddlehurst]

Also not familiar with the algorithm or whether this is a correct implementation, but would be good to discuss how to integrate series to series before putting this in probably. Thanks very much for the work though, this is helpful.

[rwtarpit]

I had a brief discussion on s2s and this linear forecaster with @hadifawaz1999 on discord. let me know whatever is decided and I refactor this accordingly.
thanks!