chore(datasets): collapse skip-timestamp-check warning to once-per-process

[shuheng-liu]

What this does

The Skipping timestamp sync check for ... warning at
src/opentau/datasets/lerobot_dataset.py:1348 previously fired
num_processes × num_datasets times — for a heterogeneous
pretraining mixture (~392 datasets) on 8 GPUs that's ~3,136 identical
lines between accelerate init and the first training step, drowning
the startup log.

skip_timestamp_check is fundamentally a mixture-wide decision
(with optional per-dataset override), and the warning text is identical
for every dataset that has it set. So the right collapse is once per
process — combined with the rank-0 gate that's one line per run,
not per dataset.

Implementation mirrors the existing _CONTROL_MODE_WARNED dedup
pattern in the same file:

• New module-level _SKIP_TIMESTAMP_WARNED: bool flag.
• Inside LeRobotDataset.__init__, the warning is gated by both
  rank (acc.is_main_process) and the flag — fire once, then flip.
• Falls through to logging.warning when no Accelerator is set so
  single-process dev / pytest runs are unchanged.
• Message now mentions the first dataset hit as an example rather
  than the only one, since the flag is mixture-wide.

How it was tested

• pre-commit run --files src/opentau/datasets/lerobot_dataset.py — clean.
• pytest -m "not gpu" -n auto tests/datasets/ — 375 passed, 7
  skipped, no regressions.

Behavior comparison:

Setup	Lines emitted (before)	Lines emitted (after)
8 ranks × 392 datasets	~3,136	1
1 rank (CPU dev) × 5 datasets	5	1
1 rank × 1 dataset	1	1

The opt-in semantic and safety implication of skip_timestamp_check
itself are unchanged — only the per-rank × per-dataset duplication
is suppressed.

How to checkout & try? (for the reviewer)

git fetch origin claude/skip-timestamp-warning-rank0
git checkout claude/skip-timestamp-warning-rank0
pytest -m "not gpu" -n auto tests/datasets/

Checklist

• I have added Google-style docstrings to important functions and ensured function parameters are typed.
• My PR includes policy-related changes.
  • If the above is checked: I have run the GPU pytests (pytest -m "gpu") and regression tests.

[claude]

[claude-review] summary for commit 043c968

No blocking issues found. The 043c968 commit addresses the prior nit by wrapping the new test body in try/finally so _SKIP_TIMESTAMP_WARNED is captured up-front and restored in the finally clause — robust to test failure and prevents cross-test bleed-through within a pytest-xdist worker if a future test wants the warning to fire again.

Confirmed correctness end-to-end:

• Module-level flag + rank-0 gate (acc is None or acc.is_main_process) at src/opentau/datasets/lerobot_dataset.py:1366 cleanly mirrors the established _CONTROL_MODE_WARNED dedup at line 1326. The acc is None fallthrough preserves single-process dev / pytest behavior.
• global _SKIP_TIMESTAMP_WARNED is correctly declared before the assignment on line 1367.
• No distributed-collective concern: each rank has its own module-level flag, only rank 0 ever logs / flips, and the flag isn't communicated across ranks (it doesn't need to be — non-rank-0 ranks short-circuit on is_main_process regardless of flag state).
• The test at tests/datasets/test_datasets.py:1030 exercises the per-process dedup with two distinct repo_ids and the try/finally save/restore is the right shape — matches the convention requested in the prior round.

[claude]

[claude-review] inline finding for commit 3a36dce

See the summary comment on this PR for the rollup.

[shuheng-liu]

@claude fix the missing test(s).

[shuheng-liu]

@claude fix the nit

[claude]

[claude-fix] Addressed in 043c968: wrapped the test body in try/finally so _SKIP_TIMESTAMP_WARNED is saved and restored to its original value, even on failure. This prevents cross-test bleed-through within the same pytest-xdist worker if a future test wants to assert the warning fires again.

tests: passed — pytest -m "not gpu" -n auto tests/datasets/test_datasets.py (42 passed; 2 pre-existing test_do_not_use_imagenet_stats failures are HF cache misses unrelated to this change).