[ENH] Add contributor extension template for v2 model implementation #2194

[sujanyd]

-> Reference Issues/PRs

Fixes #2194. See also #1993 (v2 roadmap).

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-> What does this implement/fix? Explain your changes.

This PR adds standardized extension templates for contributors implementing new models under the v2 architecture. Previously, contributors had to reverse-engineer the expected structure from existing model implementations that were themselves still being migrated.

New files added
- pytorch_forecasting/models/model_template.py
Template for subclassing BaseModel, including init, forward, and optional training_step with inline RATIONALE: comments
- pytorch_forecasting/models/pkg_template.py
Template for the model’s package container (Base_pkg), with _tags, get_cls, get_datamodule_cls, and get_test_train_params
- pytorch_forecasting/data/datamodule_template.py
Template for a custom DataModule when EncoderDecoderTimeSeriesDataModule is insufficient
- tests/test_models/test_template.py
Test template covering integration and pickle tests

Modified files
- CONTRIBUTING.md
Added Implementing New Models (v2) section with direct links to all templates

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-> Bonus enhancement (can be split into a separate PR if preferred)
- pytorch_forecasting/data/timeseries/_timeseries.py
Added interpolation_strategy parameter (forward_fill, linear, zero) to TimeSeriesDataSet
- tests/test_data/test_interpolation.py
Full test coverage for all strategies including mixed per-column strategies
- scripts/demo_interpolation.py
Visual demo comparing the three interpolation strategies

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-> What should a reviewer concentrate their feedback on?
- Are the CONTRIBUTOR TODO and RATIONALE: comments clear and sufficient for a new contributor?
- Does the model_template.py correctly reflect the v2 BaseModel API (especially metadata, _pkg, and training_step)?
- Is pkg_template.py aligned with the current Base_pkg interface?
- Should the interpolation changes in _timeseries.py be moved to a separate PR?

-> Did you add any tests for the change?
- Added tests/test_models/test_template.py
A contributor-facing test template with integration and pickle test stubs
- Added tests/test_data/test_interpolation.py
Full tests for the interpolation_strategy feature covering forward_fill, linear, zero, and mixed per-column strategies

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-> Any other comments?

This was listed as an explicit open work item in the v2 roadmap issue #1993: Add extension Templates for new contributors.

All template files follow a consistent pattern of CONTRIBUTOR TODO for actionable items and RATIONALE: for explanatory context, to make onboarding as smooth as possible.

⸻

-> PR checklist
- The PR title starts with [ENH]
- 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 independently, execute pre-commit run --all-files)

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[phoeenniixx]

Thanks a lot!

• Can you please create 2 separate files - for model and pkg.
• Please add the pointers to all the methods what they expect, what to pass etc. Look at sktime extension templates for reference.
• Add all the tags and what they mean in the pkg class

[phoeenniixx]

Do we need tests for templates as well?