[ENH] Implementing D2 data module, tests and TimeXer model from tslib for v2 (#1836)

### Description

This PR fixes issue #1833.

This PR involves the following changes:

- New dataset and datamodule for `tslib` -
[link](https://github.com/sktime/pytorch-forecasting/pull/1836/files#diff-bcb65c03ac714a1561901abf528b65d4754b327905fa590f31a15467629ef680)
- Implementation of `TimeXer` with new data module. -
[link](https://github.com/sktime/pytorch-forecasting/pull/1836/files#diff-81d7988c60658a6206368bbfd48f92ce2efb53883f0d6d52ecf4c92c68546d8b)
- Base class implementation of a `TslibBaseModel` -
[link](https://github.com/sktime/pytorch-forecasting/pull/1836/files#diff-05f662f6e4229d72f94c90fbf70cc3fa007833c69681fb18e47220c5b7df5919).
- Tests for the `tslib` data module. -
[link](https://github.com/sktime/pytorch-forecasting/pull/1836/files#diff-8a5a7aaf7fcdb0efa90272d5bdba449dabea7d25ec15321c3228dc983749e5dc)
- Example notebook -
[link](https://github.com/sktime/pytorch-forecasting/pull/1836/files#diff-1c7358b4322a7125a2145077588b15232fe2bfb7b23063267ea2638578ad8658)
- 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](https://github.com/sktime/pytorch-forecasting/pull/1836/files#diff-f2ce653a26538074931d0d28310c3f42ba52beb95ebd854739d678abb1203741)

Author: PranavBhatP

docs/source/tutorials/tslib_v2_example.ipynb

@@ -0,0 +1 @@
+"""Tests for data modules and dataloaders in pytorch_forecasting.data package."""