Fix divide_dataset in Federated Datasets

[adam-narozniak]

Issue

When specifying fraction divisions that result in more than 2 divisions (parts of the divided dataset), the latter ones (3-rd, 4-th ...) are incorrect (due to the wrong index calculation). See #3188

Proposal

• Fix the incorrect index calculation.
• Extend the tests.
• Apply the _check_division_config_correctness before the division.

Explanation

The start_idx += end_idx incorrectly does +=, it should just be reassigned start_idx = end_idx as pointed out in the issue.

Changelog entry