fix optimizer number of steps

[entrpn]

Description

Fixes the amount of optimizer steps that are set when using gradient accumulation with optax.MultiStep.

By using student_config.steps, we ensure that the maxtext optimizer, lr_scheduler and optax.MultiStep stay in sync with the difference being how many backward passes are performed based on gradient_accumulation. Ex:

• ga=1, 5k steps, will do 5k backward passes.
• ga=2, 5k steps, will do 5k/2 backward passes.

The number of samples seen by the model stay exactly the same.

Tests

Ran:

• tests.unit.distillation_checkpointing_test
• tests.unit.train_distill_test

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[codecov]

Codecov Report

❌ Patch coverage is 36.69065% with 88 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/maxtext/models/deepseek_batchsplit.py	6.97%	40 Missing ⚠️
src/maxtext/trainers/tokenizer/train_tokenizer.py	45.16%	28 Missing and 6 partials ⚠️
src/maxtext/layers/quantizations.py	60.60%	12 Missing and 1 partial ⚠️
.../trainers/post_train/distillation/train_distill.py	0.00%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!