implementing changes for exposing Lightning Tuner's LR finder (#1609)

* implementing changes for exposing Lightning Tuner's LR finder

* decouple dataset/loader generation from training

* improve decoupling

* add unit test and docs

* add plotting from unittest

* remove lr_finder example

* fix minor mistakes in docs

* final updates for PR

* Update darts/tests/models/forecasting/test_torch_forecasting_model.py

Co-authored-by: madtoinou <[email protected]>

* fix Tuner import for PTL<2.0.0

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Co-authored-by: Dennis Bader <[email protected]>
Co-authored-by: madtoinou <[email protected]>

Author: 3 people