Support folder-based datasets with large metadata.jsonl

[gbenson]

I tried creating an imagefolder dataset with a 714MB metadata.jsonl but got the error below. This pull request fixes the problem by increasing the block size like the message suggests.

>>> from datasets import load_dataset
>>> dataset = load_dataset("imagefolder", data_dir="data-for-upload")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/path/to/datasets/load.py", line 2609, in load_dataset
    builder_instance.download_and_prepare(
      ...
  File "/path/to/datasets/packaged_modules/folder_based_builder/folder_based_builder.py", line 245, in _read_metadata
    return paj.read_json(f)
  File "pyarrow/_json.pyx", line 308, in pyarrow._json.read_json
  File "pyarrow/error.pxi", line 154, in pyarrow.lib.pyarrow_internal_check_status
  File "pyarrow/error.pxi", line 91, in pyarrow.lib.check_status
pyarrow.lib.ArrowInvalid: straddling object straddles two block boundaries (try to increase block size?)