Add optional batch_size parameter to BLEURT compute method

[aakash-agarwal-002]

This PR updates the BLEURT metric implementation to support an optional batch_size argument in the compute method.

Changes made:

• Added a batch_size=None parameter to _compute.
• Passed batch_size through to self.scorer.score (batch_size parameter is also accepted by https://github.com/google-research/bleurt/blob/master/bleurt/score.py).
• Updated usage examples to include batch_size=32