🪡 Various RLOO fixes

[qgallouedec]

What does this PR do?

Closes #2316

• Remove vf_loss_stats (not used)
• Remove unnecessary global step increment (same as [ODPO] Fix global step for consistent checkpointing with global updates #1950)
• Fix epoch calculation (plot before after)

python examples/scripts/rloo/rloo.py
     --dataset_name trl-internal-testing/descriptiveness-sentiment-trl-style
     --dataset_train_split descriptiveness
     --learning_rate 3e-6
     --num_ppo_epochs 1
     --num_mini_batches 1
     --output_dir models/minimal/ppo
     --per_device_train_batch_size 64
     --gradient_accumulation_steps 1
     --total_episodes 10000
     --model_name_or_path EleutherAI/pythia-1b-deduped
     --missing_eos_penalty 1.0

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