GitLab

* fix 5990

* accomodate iterable dataset without predefined length
* set it as 1 use case: provide max_steps, and NO num_epochs
* Is a merge of master and PR 5995

* fix trainer test under TF

* fix only for torch
* TF trainer untouched
* trainer tests are skipped when no torch

* address comments

* fix quality checks

* remove torch.dataset from test_trainer

* unnecessary inheritance
* RegressionDataset implements all needed methods __len__ and __getitem__

* fix quality checks

* restore RegressionDataset

* was wrongly under is_torch_available()