[ASR] Add unit tests for VAD thresholding and post-processing logic

[shivansh023023]

What does this PR do?

Adds 26 CPU-only unit tests for the VAD (Voice Activity Detection) thresholding
and post-processing pipeline in nemo/collections/asr/parts/utils/vad_utils.py.
These functions had zero test coverage prior to this PR.

Collection: ASR

Changelog

• tests/collections/asr/test_vad_utils.py (new file, 221 lines, 26 tests):
  • TestBinarization: symmetric threshold, hysteresis, empty input, all-zeros, all-ones, single-frame edge cases
  • TestBinarizationParametrized: threshold sweep over onset in [0.3, 0.5, 0.7, 0.9]
  • TestFiltering: short segment removal, no-op filtering, empty input
  • TestMergeOverlapSegment: overlapping merge, non-overlapping, empty input
  • TestSegmentHelpers: filter_short_segments, get_gap_segments, remove_segments
  • TestCalVadOnsetOffset: absolute and relative scale conversion
  • TestPercentile: basic and boundary percentile values
  • TestGenerateVadSegmentTablePerTensor: full pipeline correctness, determinism, non-overlapping output

Usage

# Run the new tests locally
pytest tests/collections/asr/test_vad_utils.py -v
# All 26 tests pass on CPU with no GPU required

Before your PR is "Ready for review"

Pre checks:

• Make sure you read and followed Contributor guidelines
• Did you write any new necessary tests? — Yes, this PR IS the tests
• Did you add or update any necessary documentation? — No docs changes needed for test-only PR
• Does the PR affect components that are optional to install? — No

PR Type:

• New Feature
• Bugfix
• Documentation / Tests

Additional Information

• All 26 tests are CPU-only and fully deterministic via torch.manual_seed(42)
• No existing files were modified — new test file only
• Follows the same class-based pytest structure as PR [TTS] Add comprehensive MoE dispatch unit tests with golden-value regression checks #15698

[copy-pr-bot]

This pull request requires additional validation before any workflows can run on NVIDIA's runners.

Pull request vetters can view their responsibilities here.

Contributors can view more details about this message here.