PPOTrainer: fix progress bar for num_mini_batches > 1

[dawidm]

What does this PR do?

Simplest approach to fix #2530

It changes convention of:

• batch size, which is now number of samples in main PPO loop (previously in mini-batch PPO loop). This changes behavior only when args.num_mini_batches > 1 (configurations that had bugged progress bar before this fix).
• step, which is now equivalent of episodes, which is consistent with documentation (https://huggingface.co/docs/trl/main/en/ppo_trainer#explanation-of-the-logged-metrics): ''episode: episode: The current global step or episode count in the training process.''.

If this gets accepted I will modify my PR for RLOO #2516 to follow the same convention.

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