Add ragged all-to-all pipeline correctness coverage

[huytransformer]

Description

Adds a TPU regression test for DeepSeek MoE pipeline parallelism with EP ragged A2A.

b/418313093

Tests

Ran this:

python3 -m pytest -q tests/integration/pipeline_parallelism_test.py \
  -k test_deepseek_ragged_a2a_ep_same_output_and_grad -s

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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