Add PITS imputation model

[Copilot]

• Create pypots/nn/modules/pits/ with PITS backbone layers (PITSBlock, PITSBackbone - patch-independent processing without cross-patch attention)
• Create pypots/imputation/pits/ with core and model files
• Update pypots/imputation/__init__.py to export PITS
• Create tests/imputation/pits.py test file (test_0_fit, test_1_impute, test_2_parameters, test_3_saving_path, test_4_lazy_loading)
• Fix .gitignore to allow tracking new test files while excluding pycache
• Update docs/algo_table.rst with PITS entry
• Update docs/references.bib with PITS citation
• Update docs/pypots.imputation.rst with PITS module documentation
• Update docs/pypots.nn.rst with PITS nn module documentation

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add PITS</issue_title>
<issue_description>### 1. Model description

PITS should be included in PyPOTS, and could start from the imputation task.

@inproceedings{lee2024pits,
title={Learning to Embed Time Series Patches Independently},
author={Seunghan Lee and Taeyoung Park and Kibok Lee},
booktitle={The Twelfth International Conference on Learning Representations},
year={2024},
url={https://openreview.net/forum?id=WS7GuBDFa2}
}

2. Check open-source status

• The model implementation is publicly available

3. Provide useful information for the implementation

https://github.com/seunghan96/pits</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add PITS #392

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[WenjieDu]

@copilot The unit testing file tests/imputation/pits.py is missing. Add it and implement the testing cases.

[Copilot]

@copilot The unit testing file tests/imputation/pits.py is missing. Add it and implement the testing cases.

Added tests/imputation/pits.py in commit 2708e22. The file was missing from the initial commit because the .gitignore rule *test* was blocking new files under tests/ from being tracked (existing test files were already tracked and unaffected). Fixed by adding !tests/ and !tests/** negation rules to .gitignore (cf3acae also cleans up accidentally committed pycache files).

[sonarqubecloud]

Quality Gate failed

Failed conditions
27.4% Duplication on New Code (required ≤ 3%)

See analysis details on SonarQube Cloud

[github-actions]

This pull request had no activity for 14 days. It will be closed in 1 week unless there is some new activity.