Fix/8877 elastic transform negative sigma

[AayushMainali-Github]

Summary

• Reject negative sigma when constructing v1 and v2 ElasticTransform, matching the existing “sigma should be positive/non-negative” expectation.
• Keep sigma=0 valid so smoothing can still be disabled along an axis.
• Add regression tests for scalar negatives, all-negative sequences, and mixed-sign sequences.

Fixes #8877

Problem

ElasticTransform only runs gaussian_blur when sigma[i] > 0. All-negative inputs such as sigma=-100 or sigma=[-100, -100] therefore skip the blur and never hit the kernel’s positivity check. Mixed-sign values still fail later, which made the constructor validation look inconsistent.

Fix

After sigma is normalized to a two-element sequence, both constructors raise ValueError if any component is negative. Zero remains allowed because the displacement code already treats sigma == 0 as “do not smooth.”

Testing

• python -m pytest test/test_transforms.py::test_elastic_transformation test/test_transforms_v2.py::TestElastic::test_transform_negative_sigma -q (6 passed)
• python -m pytest test/test_transforms_v2.py::TestElastic -q -k "not test_kernel and not test_functional" (33 passed, 21 skipped on CPU-only)
• Confirmed the new tests fail with DID NOT RAISE ValueError on unmodified constructors
• ufmt check on the changed files passed

[pytorch-bot]

🔗 Helpful Links

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