[BUG]: Fixed the incorrect time propagation

[Chetansahney]

Reference Issues/PRs

Fixes #2263

What does this fix? Explain your changes.

This fixes an issue where history_time_idx and future_time_idx in pytorch_forecasting/data/_tslib_data_module.py could be generated from synthetic sequential indices (torch.arange(...)) instead of real timesteps.
The implementation now propagates true timesteps from processed_data["timestep"], preserving non-zero and irregular time indices.

What should a reviewer concentrate their feedback on?

• Correctness of history_time_idx / future_time_idx construction from processed_data["timestep"]
• Any side effects on window slicing/batching behavior in TSLib v2
• Regression test coverage for irregular/non-zero time index scenarios

Did you add any tests for the change?

Yes. Added:
pytorch_forecasting/data/tests/test_tslib_data_module.py::test_tslib_dataset_preserves_original_time_idx

Also validated with:

• ruff check on changed files

Any other comments?

There is a pre-existing unrelated baseline failure in full test_tslib_data_module.py (test_multivariate_target, list-vs-shape assertion), observed prior to this fix.

PR checklist

• The PR title starts with either [ENH], [MNT], [DOC], or [BUG]. [BUG] - bugfix, [MNT] - CI, test framework, [ENH] - adding or improving code, [DOC] - writing or improving documentation or docstrings.
• 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

[codecov]

Codecov Report

❌ Patch coverage is 55.00000% with 9 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@818a84d). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...h_forecasting/data/tests/test_tslib_data_module.py	0.00%	8 Missing ⚠️
pytorch_forecasting/data/_tslib_data_module.py	91.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2264   +/-   ##
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  Coverage        ?   86.93%           
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  Files           ?      166           
  Lines           ?     9783           
  Branches        ?        0           
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  Hits            ?     8505           
  Misses          ?     1278           
  Partials        ?        0           

Flag	Coverage Δ
cpu	86.93% <55.00%> (?)
pytest	86.93% <55.00%> (?)

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