The max number of response protos to put into each output JSON
file on Google Cloud Storage. 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.