Increase tolerance in TestGARCH11::test_batched_size to reduce flakiness

[JigyasuRajput]

Description

Increased the absolute (atol=1e-3) and relative (rtol=1e-2) tolerances in np.testing.assert_allclose within TestGARCH11::test_batched_size.
This should help reduce test flakiness caused by minor numerical variations.

Related Issue

• [x] Closes #7674

Checklist

• [x] Checked that the pre-commit linting/style checks pass
• [x] Included tests that prove the fix is effective or that the new feature works (ran pytest tests/distributions/test_timeseries.py)
• [ ] Added necessary documentation (docstrings and/or example notebooks)
• [x] If you are a pro: each commit corresponds to a relevant logical change

Type of change

• [x] Bug fix
• [ ] New feature / enhancement
• [ ] Documentation
• [ ] Maintenance
• [ ] Other (please specify):

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📚 Documentation preview 📚: https://pymc--7684.org.readthedocs.build/en/7684/