Move RL code from src/MaxText/rl/ to src/maxtext/trainers/post_train/rl/

[A9isha]

Description

Migrate RL training code to the new package structure following the same pattern as the SFT (PR #2988) and distillation moves. Old location files are replaced with backward-compatibility shims that delegate to the new modules with deprecation warnings.

Also, fixing the Jupyter notebook run test but disabling sft_qwen3_demo.ipynb for further investigation

Tests

Locally ran RL using the following commands:

## new
python3 -m src.maxtext.trainers.post_train.rl.train_rl src/maxtext/configs/post_train/rl.yml   model_name=llama3.1-8b   t
okenizer_path=meta-llama/Llama-3.1-8B-Instruct   load_parameters_path=/path/to/checkpoint   run_name=maz-8b-$RANDOM   bas
e_output_directory=/path/to/storage   hf_access_token=<HF_TOKEN> dataset_name=gsm8k steps=4


## old
python3 -m src.MaxText.rl.train_rl src/maxtext/configs/post_train/rl.yml   model_name=llama3.1-8b   t
okenizer_path=meta-llama/Llama-3.1-8B-Instruct   load_parameters_path=/path/to/checkpoint   run_name=maz-8b-$RANDOM   bas
e_output_directory=/path/to/storage   hf_access_token=<HF_TOKEN> dataset_name=gsm8k steps=4

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[bvandermoon]

For the manual tests you ran, could you also try with the old commands? Not as critical as the train.py shims since RL is a newer feature, but still good to test them since they are added here

[A9isha]

Done testing with the old command and updated the description - thanks @bvandermoon

[bvandermoon]

Done testing with the old command and updated the description - thanks @bvandermoon

Thanks @A9isha. Just to double check, can you confirm you saw all logs as expected with the old command? For train.py, I needed to set logging.set_verbosity(logging.INFO) to see the standard completed step output logged