The max number of response protos to put into each output
JSON file on GCS. The valid range is [1, 100]. If not
specified, the default value is 20.
For example, for one pdf file with 100 pages, 100 response
protos will be generated. If ``batch_size`` = 20, then 5
json files each containing 20 response protos will be
written under the prefix ``gcs_destination``.\ ``uri``.
Currently, batch_size only applies to GcsDestination, with
potential future support for other output configurations.