Fix SFT + GA (gradient accumulation)

[gobbleturk]

Description

Fixes an issue where gradient accumulation + sft would fail due to missing param sharding

FIXES: b/472309528

Tests

Added a unit test (integration test) for sft + ga

Manually tested via

python3 -m MaxText.sft_trainer MaxText/configs/base.yml run_name=mattdavidow-train-base base_output_directory=$output_dir dataset_path=$dataset steps=5 enable_checkpointing=False enable_goodput_recording=False use_sft=True gradient_accumulation_steps=5

Checklist

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• 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 0% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/MaxText/sft_trainer.py	0.00%	2 Missing ⚠️

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