Fix autotrain output directory: cd to checkpoints dir before training

[Copilot]

autotrain saves models to {cwd}/{project_name}/, ignoring params.output_dir. The SLURM script ran from an unspecified directory, so models were saved elsewhere.

Changes

• train_smollm.slurm: cd to checkpoints directory before running autotrain
• config_smollm3.yaml: Added documentation explaining autotrain's output behavior

# Before: autotrain runs from unspecified cwd
autotrain --config "$CONFIG_PATH"

# After: cd to parent of expected output dir first
mkdir -p "$(dirname "$CHECKPOINT_DIR")"
cd "$(dirname "$CHECKPOINT_DIR")"
autotrain --config "$CONFIG_PATH"
# Creates: {checkpoints_dir}/{project_name}/

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