[ENH] Implementing D2 data module, tests and TimeXer model from tslib for v2

[PranavBhatP]

Description

This PR fixes issue #1833.

This PR involves the following changes:

• New dataset and datamodule for tslib - link
• Implementation of TimeXer with new data module. - link
• Base class implementation of a TslibBaseModel - link.
• Tests for the tslib data module. - link
• Example notebook - link
• Restructure codebase to include a layers directory for module containing architectural deep learning layer classes.
• Metadata container for _timexer.py using the v2 framework - link

Checklist

• Linked issues (if existing)
• Amended changelog for large changes (and added myself there as contributor)
• Added/modified tests
• Used pre-commit hooks when committing to ensure that code is compliant with hooks. Install hooks with pre-commit install.
  To run hooks independent of commit, execute pre-commit run --all-files

Make sure to have fun coding!

[PranavBhatP]

May I suggest all the files refactored for v2 to be kept separate and made private (with an underscore _)

I made changes where I could, do lmk if you want other changes.

[PranavBhatP]

@phoeenniixx , I have added timexer v2 metadata but it doesn't seem to be running the tests when I try running test_all_estimators_v2.py. Any thoughts on why this happening, you can maybe review the code?

[PranavBhatP]

@fkiraly @agobbifbk is it good practice to pass a torch.zero tensor in place of the empty history_target for a cold-start forecasting scenario i.e when there is no history_target in the input? I was considering whether this situation should be included in TimeXer. Currently, we are prohibiting a user from entering an empty history_target using checks in the D2, but I was just curious about this special case.

[agobbifbk]

Yes it seems reasonable. Sometimes you don't want to do autoregressive models. I think I need to port this feature in some of the model of DSIPTS, I think that some of them can break if I turn of the autoregressive part :-) Thank you for pointing it!