[Gemma4] Investigate flaky test_generation_beyond_sliding_window_1_eager (#48236)

* [Gemma4] Isolate test_generation_beyond_sliding_window in integration test class

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* [Gemma4] Keep only eager sliding window test for investigation

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* [Gemma4] Log full output_text before assertion for CI visibility

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* [Gemma4] Run sliding window test 10x in fresh subprocesses to expose non-determinism

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* [Gemma4] Log stdout/stderr from each subprocess run

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* [Gemma4] Remove image processing and processing tests; tweak modeling test

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* try

* try

* [Gemma4] Log free -g before subprocess loop

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* [Gemma4] Add commented-out torch.use_deterministic_algorithms for reference

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* [Gemma4] Use torch.use_deterministic_algorithms to fix flaky sliding window test

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix

* [Gemma4] Use require_deterministic_for_accelerator decorator; add it to testing_utils

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* [Gemma4] Add require_deterministic_for_accelerator; apply to sliding window test

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* Revert utils/pr_slow_ci_models.py to main

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* [Gemma4] Fix decorator order: @parameterized.expand must be outermost

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* [Gemma4] Update eager expected value after torch.use_deterministic_algorithms

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* [Gemma4] Skip test_sdpa_can_dispatch_on_flash; add TODO for modeling fix

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* [gemma4] Update stale expected value for test_model_text_only_multigpu on sm86

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* [gemma4] Clarify comment: block-overlay mask forces materialization by design

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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Co-authored-by: ydshieh <ydshieh@users.noreply.github.com>