[GRPO] Make dataloader deterministic

[daniellepintz]

This will make the dataloader deliver the data in the same order each time.

Test Plan

python -m apps.grpo.main --config apps/grpo/qwen3_1_7b.yaml

Added a breakpoint, and printed out the prompt in pdb:

Before, prompt was different across runs; after this PR prompt is the same across runs.

[casteryh]

LGTM. can you verify that this makes the dataloader deterministic beyond the first epoch too? you can test it by truncating the dataset for example.