Can't convert my dataset to Tar
Trying to convert my own dataset to Tarred dataset

the number of wav files is almost 280k.

```
python ${NEMO_ROOT}/scripts/speech_recognition/convert_to_tarred_audio_dataset.py \
  --manifest_path=train_decoded_processed.json \
  --target_dir=train_tarred_6bk \
  --num_shards=1024 \
  --max_duration=20.0 \
  --min_duration=1.0 \
  --shuffle \
  --shuffle_seed=1 \
  --sort_in_shards \
  --workers=-1 \
  --buckets_num=6
```

but this would throw the exception

![image](https://user-images.githubusercontent.com/1567200/201321512-e32e1734-a80b-4647-addc-9e96493b0dcf.png)

single bucket is fine, but failed to create multiple buckets.

any idea?