pad_to_multiple_of added to DataCollatorForWholeWordMask
There is a small bug in `DataCollatorForWholeWordMask`: it has an argument `pad_to_multiple_of`, however, when doing `_collate_batch` inside `__call__` method, this argument is not provided. This commit adds the usage of the argument.
Have checked it, works fine, both `batch_input` and `batch_mask` are padded to a multiple of `pad_to_multiple_of` value.<|||||>Ok, will do that some of these days!