Skip GCS-dependent tokenizer unit tests under decoupled offline mode

[darisoy]

Description

This PR adds code-level skip decorators to TrainTokenizerTest, TikTokenTest, and HFTokenizerTest in tests/unit/tokenizer_test.py when running in decoupled offline mode (DECOUPLE_GCLOUD=TRUE).

Problem & Context

These unit tests are hardcoded to train and verify tokenizers on GCS parquet datasets (gs://maxtext-dataset/...) and copy models using the gcloud CLI in a subprocess.

In an air-gapped GKE container running offline:

• GCS access is stubbed out, causing FileNotFoundError when JAX tries to glob the buckets.
• The minimal container image lacks the Google Cloud SDK (gcloud CLI), causing FileNotFoundError: 'gcloud' when spawning the download subprocess.

Solution

Bypass these cloud-dependent tests dynamically using @unittest.skipIf(is_decoupled(), ...) when offline decoupled mode is enabled. This allows the decoupled test suite to execute completely green out-of-the-box in offline environments, without affecting standard online runs.

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Tests

This change has been fully verified on a physical TPU7x VM slice.

Commands to Reproduce & Verify:

export DECOUPLE_GCLOUD=TRUE
python3 -m pytest tests/unit/tokenizer_test.py -vv

Expected Output:
All 5 collection errors are resolved and the classes are cleanly skipped:

tests/unit/tokenizer_test.py::TrainTokenizerTest SKIPPED
tests/unit/tokenizer_test.py::TikTokenTest SKIPPED
tests/unit/tokenizer_test.py::HFTokenizerTest SKIPPED

Checklist

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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