TypeError: 'NoneType' object does not support item assignment
command：
```
def load_datasets(formats, data_dir=datadir, data_files=datafile）：
      dataset = load_dataset(formats, data_dir=datadir, data_files=datafile, split=split, streaming=True, **kwargs)
return dataset
raw_datasets = DatasetDict()
raw_datasets["train"] = load_datasets(“csv”,  args.datadir, "train.csv", split=train_split)
raw_datasets["test"] = load_datasets(“csv”, args.datadir, "dev.csv", split=test_split)
raw_datasets = raw_datasets.cast_column("audio", Audio(sampling_rate=16000))
```

error：
```
    main()
  File "peft_adalora_whisper_large_training.py", line 502, in main
    raw_datasets = raw_datasets.cast_column("audio", Audio(sampling_rate=16000))
  File "/home/ybZhang/miniconda3/envs/whister/lib/python3.8/site-packages/datasets/dataset_dict.py", line 2015, in cast_column
    info.features[column] = feature
TypeError: 'NoneType' object does not support item assignment
```
