fix(train): show resumed position in epoch progress bar

[imargulis]

Summary

Follow-up to #603, which added mid-epoch checkpoint save and resume.

The fast-skip resume path removes already completed batches from the training
loader. Because tqdm inferred the reduced iterable length, the progress bar starts
from zero even though training had resumed mid-epoch.

Initialize the rank-zero progress bar with the full epoch total and the resumed
local step so it displays the correct percentage and remaining work.

Also adds a regression test covering the progress-bar arguments for a fully
skipped resumed epoch.

Testing

• tests/unit/train/test_mid_epoch_resume.py

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📝 Walkthrough

Walkthrough

Changes

Mid-epoch progress reporting

Layer / File(s)	Summary
Initialize progress at the resume step src/speculators/train/trainer.py, tests/unit/train/test_mid_epoch_resume.py	Trainer.train_epoch initializes tqdm with the epoch total and skipped-step offset; a unit test verifies the progress bar arguments during mid-epoch resume.

Possibly related PRs

• vllm-project/speculators#603: Adds related mid-epoch fast-skip resume behavior used by the progress-bar initialization.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: showing the resumed position in the epoch progress bar.
Description check	✅ Passed	The description matches the implemented fix and regression test for mid-epoch resume progress-bar behavior.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@src/speculators/train/trainer.py`:
- Around line 432-436: In src/speculators/train/trainer.py lines 432-436, keep
the requested resume offset separate from the number of batches actually skipped
by the fast-skip path, and pass the actual skipped count to tqdm’s initial value
so fallback replay starts at zero. In tests/unit/train/test_mid_epoch_resume.py
lines 408-448, add coverage using a sampler without fast-skip that verifies
progress initialization remains zero during replay.

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📥 Commits

Reviewing files that changed from the base of the PR and between a9bc119 and 5f624d5.

📒 Files selected for processing (2)

• src/speculators/train/trainer.py
• tests/unit/train/test_mid_epoch_resume.py

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