Shrink contrastive grader test data

[RussTedrake]

Summary

• Reduce the contrastive grader fixture from 480×640×128 to 128×160×32.
• Keep the 4,096 sample pairs and grader assertions unchanged.

Validation

• poetry install --all-extras --with=dev,docs
• .venv/bin/python -m pytest -q book/deep_perception/exercises/test_notebooks_deep_perception_exercises.py solutions/notebooks/deep_perception/test_notebooks_solutions_deep_perception.py (2 passed)
• Pre-commit hooks passed.

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[RussTedrake]

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