Add a GSM8K SFT dataset loader

[adohe]

Scope

• Add examples/math/sft_dataset_loader.py that converts raw GSM8K into an SFT-ready prompt+response schema (distinct from the RL-shaped examples/math/dataset_loader.py).
• Wire it as the --dataset-loader-fn example used by the SFT dataset tutorial.

Acceptance

• Output rows are valid SFT input (prompt+response).
• Does not alter the existing RL loader.
• Include a small CPU test asserting the produced schema.