Adjust padding in batch generation

[gaetanlop]

What does this PR do?

Fix a bug in the batch_generation function where we try to concatenate tensors that can be of different dimensions (sequence_length) when generating with mini_batches.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
• Did you write any new necessary tests?

Who can review?

@kashif @qgallouedec