Checksum Failure when Downloading Datasets from Seqio Mixture

Short description
In a Google Research repository, several users are hitting an issue where they get checksum errors for TFDS datasets they need to download (e.g. see bottom of this thread), because the checksum has changed. It isn't clear to me how this is intended to be avoided.

Environment information

• Operating System: Mac OS

• Python version: 3.7

• tensorflow-datasets/tfds-nightly version: 4.4.0.dev202108200109

• tensorflow/tf-nightly version: 2.6.0

• Does the issue still exists with the last tfds-nightly package (pip install --upgrade tfds-nightly) ?

• Yes

Reproduction instructions

Clone this Google repository. Then in python flan/v2/run_example.py comment in line 86.

pip install --upgrade pip
pip install -r flan/v2/requirements.txt
PYTHONPATH=. python flan/v2/run_example.py

If you share a colab, make sure to update the permissions to share it.

Link to logs

ERROR:absl:Failed to load task 'aeslc_template_0to10_no_opt_x_shot' as part of mixture 'flan2022_submix'
Traceback (most recent call last):
  File "/home/henry/flan/FLAN/flan/v2/run_example.py", line 93, in <module>
    dataset = selected_mixture.get_dataset(
  File "/home/henry/anaconda3/envs/palm/lib/python3.9/site-packages/seqio/dataset_providers.py", line 1730, in get_dataset
    ds = task.get_dataset(
  File "/home/henry/anaconda3/envs/palm/lib/python3.9/site-packages/seqio/dataset_providers.py", line 1375, in get_dataset
    ds = source.get_dataset(
  File "/home/henry/anaconda3/envs/palm/lib/python3.9/site-packages/seqio/experimental.py", line 369, in get_dataset
    train_ds = _get_maybe_sharded_dataset(
  File "/home/henry/anaconda3/envs/palm/lib/python3.9/site-packages/seqio/experimental.py", line 329, in _get_maybe_sharded_dataset
    num_shards = len(self._original_source.list_shards(split_))
  File "/home/henry/anaconda3/envs/palm/lib/python3.9/site-packages/seqio/dataset_providers.py", line 510, in list_shards
    return [_get_filename(info) for info in self.tfds_dataset.files(split)]
  File "/home/henry/anaconda3/envs/palm/lib/python3.9/site-packages/seqio/utils.py", line 159, in files
    split_info = self.builder.info.splits[split]
  File "/home/henry/anaconda3/envs/palm/lib/python3.9/site-packages/tensorflow_datasets/core/splits.py", line 391, in __getitem__
    raise KeyError(
KeyError: "Trying to access `splits['train']` but `splits` is empty. This likely indicate the dataset has not been generated yet."

Expected behavior
To download TFDS dataset successfully.