Add tqdm progress bar to RayPPOTrainer for training visualization

This PR enhances the RayPPOTrainer class by implementing a progress bar
that visualizes the training process:

- Imported tqdm module in verl/trainer/ppo/ray_trainer.py (line 27)
- Added progress bar initialization in the fit() method (line 781)
- Implemented progress updates during training iterations (line 931)
- Added proper cleanup by closing the progress bar at the end of
training (line 928)

This improvement provides real-time feedback on training progress,
making it easier to monitor long-running training sessions.

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Co-authored-by: hoshi-hiyouga <hiyouga@buaa.edu.cn>