audio column not saved correctly after resampling
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['Hi ! We just released a new version of `datasets` that should fix this.\r\n\r\nI tested resampling and using save/load_from_disk afterwards and it seems to be fixed now'
 'Hi @lhoestq, \r\n\r\nJust tested the latest datasets version, and confirming that this is fixed for me. \r\n\r\nThanks!'
 'Also, just an FYI, data that I had saved (with save_to_disk) previously from common voice using datasets==1.17.0 now give the error below when loading (with load_from disk) using datasets==1.18.0. \r\n\r\nHowever, when starting fresh using load_dataset, then doing the resampling, the save/load_from disk worked fine. \r\n\r\n```\r\n---------------------------------------------------------------------------\r\nValueError                                Traceback (most recent call last)\r\n<timed exec> in <module>\r\n\r\n/opt/conda/lib/python3.7/site-packages/datasets/load.py in load_from_disk(dataset_path, fs, keep_in_memory)\r\n   1747         return Dataset.load_from_disk(dataset_path, fs, keep_in_memory=keep_in_memory)\r\n   1748     elif fs.isfile(Path(dest_dataset_path, config.DATASETDICT_JSON_FILENAME).as_posix()):\r\n-> 1749         return DatasetDict.load_from_disk(dataset_path, fs, keep_in_memory=keep_in_memory)\r\n   1750     else:\r\n   1751         raise FileNotFoundError(\r\n\r\n/opt/conda/lib/python3.7/site-packages/datasets/dataset_dict.py in load_from_disk(dataset_dict_path, fs, keep_in_memory)\r\n    769                 else Path(dest_dataset_dict_path, k).as_posix()\r\n    770             )\r\n--> 771             dataset_dict[k] = Dataset.load_from_disk(dataset_dict_split_path, fs, keep_in_memory=keep_in_memory)\r\n    772         return dataset_dict\r\n    773 \r\n\r\n/opt/conda/lib/python3.7/site-packages/datasets/arrow_dataset.py in load_from_disk(dataset_path, fs, keep_in_memory)\r\n   1118             info=dataset_info,\r\n   1119             split=split,\r\n-> 1120             fingerprint=state["_fingerprint"],\r\n   1121         )\r\n   1122 \r\n\r\n/opt/conda/lib/python3.7/site-packages/datasets/arrow_dataset.py in __init__(self, arrow_table, info, split, indices_table, fingerprint)\r\n    655         if self.info.features.type != inferred_features.type:\r\n    656             raise ValueError(\r\n--> 657                 f"External features info don\'t match the dataset:\\nGot\\n{self.info.features}\\nwith type\\n{self.info.features.type}\\n\\nbut expected something like\\n{inferred_features}\\nwith type\\n{inferred_features.type}"\r\n    658             )\r\n    659 \r\n\r\nValueError: External features info don\'t match the dataset:\r\nGot\r\n{\'accent\': Value(dtype=\'string\', id=None), \'age\': Value(dtype=\'string\', id=None), \'audio\': Audio(sampling_rate=48000, mono=True, id=None), \'client_id\': Value(dtype=\'string\', id=None), \'down_votes\': Value(dtype=\'int64\', id=None), \'gender\': Value(dtype=\'string\', id=None), \'locale\': Value(dtype=\'string\', id=None), \'path\': Value(dtype=\'string\', id=None), \'segment\': Value(dtype=\'string\', id=None), \'sentence\': Value(dtype=\'string\', id=None), \'up_votes\': Value(dtype=\'int64\', id=None)}\r\nwith type\r\nstruct<accent: string, age: string, audio: struct<bytes: binary, path: string>, client_id: string, down_votes: int64, gender: string, locale: string, path: string, segment: string, sentence: string, up_votes: int64>\r\n\r\nbut expected something like\r\n{\'accent\': Value(dtype=\'string\', id=None), \'age\': Value(dtype=\'string\', id=None), \'audio\': {\'path\': Value(dtype=\'string\', id=None), \'bytes\': Value(dtype=\'binary\', id=None)}, \'client_id\': Value(dtype=\'string\', id=None), \'down_votes\': Value(dtype=\'int64\', id=None), \'gender\': Value(dtype=\'string\', id=None), \'locale\': Value(dtype=\'string\', id=None), \'path\': Value(dtype=\'string\', id=None), \'segment\': Value(dtype=\'string\', id=None), \'sentence\': Value(dtype=\'string\', id=None), \'up_votes\': Value(dtype=\'int64\', id=None)}\r\nwith type\r\nstruct<accent: string, age: string, audio: struct<path: string, bytes: binary>, client_id: string, down_votes: int64, gender: string, locale: string, path: string, segment: string, sentence: string, up_votes: int64> \r\n```']
## Describe the bug
After resampling the audio column, saving with save_to_disk doesn't seem to save with the correct type. 

## Steps to reproduce the bug
- load a subset of common voice dataset (48Khz)
- resample audio column to 16Khz
- save with save_to_disk()
- load with load_from_disk()

## Expected results
I expected that after saving the data, and then loading it back in, the audio column has the correct dataset.Audio type (i.e. same as before saving it)
{'accent': Value(dtype='string', id=None),
 'age': Value(dtype='string', id=None),
 'audio': Audio(sampling_rate=16000, mono=True, _storage_dtype='string', id=None),
 'client_id': Value(dtype='string', id=None),
 'down_votes': Value(dtype='int64', id=None),
 'gender': Value(dtype='string', id=None),
 'locale': Value(dtype='string', id=None),
 'path': Value(dtype='string', id=None),
 'segment': Value(dtype='string', id=None),
 'sentence': Value(dtype='string', id=None),
 'up_votes': Value(dtype='int64', id=None)}

## Actual results
Audio column does not have the right type
{'accent': Value(dtype='string', id=None),
 'age': Value(dtype='string', id=None),
 'audio': {'bytes': Value(dtype='binary', id=None),
  'path': Value(dtype='string', id=None)},
 'client_id': Value(dtype='string', id=None),
 'down_votes': Value(dtype='int64', id=None),
 'gender': Value(dtype='string', id=None),
 'locale': Value(dtype='string', id=None),
 'path': Value(dtype='string', id=None),
 'segment': Value(dtype='string', id=None),
 'sentence': Value(dtype='string', id=None),
 'up_votes': Value(dtype='int64', id=None)}

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 1.17.0
- Platform: linux
- Python version:
- PyArrow version:

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