Pad partial file prompt batches

[ikarth]

If the number of prompts is not a multiple of the size of the batch, it causes errors.

To avoid this, we can run the final prompt some additional times to pad out the last batch.

This is just one possible solution to the issue, of course.

[ikarth]

(temporarily closed the pull request when I discovered another edge case my code didn't handle.)

[ikarth]

(Fixed my math error.)

[GoMapur]

Thank you for fixing this my friend :)