can't use dali with comma seperated manifest
When using nvidia dali to train speech to text,

```python

cfg = copy.deepcopy(char_model.cfg)

"""
read cleaned manifest
"""
train_manifest_data = read_manifest(train_manifest_cleaned)
train_charset = get_charset(train_manifest_data)
dev_manifest_data = read_manifest(dev_manifest_cleaned)
dev_charset = get_charset(dev_manifest_data)

train_dev_set = set.union(set(train_charset.keys()), set(dev_charset.keys()))



# Setup train, validation, test configs
with open_dict(cfg):
  # Train dataset  (Concatenate train manifest cleaned and dev manifest cleaned)
  cfg.train_ds.manifest_filepath = f'{train_manifest_cleaned},{dev_manifest_cleaned}'
  cfg.train_ds.labels = list(train_dev_set)
  cfg.train_ds.normalize_transcripts = False
  cg.train_ds.batch_size = 32
  cfg.train_ds.use_dali = True
  cfg.train_ds.num_workers = 8
  cfg.train_ds.pin_memory = True
  cfg.train_ds.trim_silence = True

char_model.setup_training_data(cfg.train_ds)
```

![image](https://user-images.githubusercontent.com/1567200/200490593-dfa9ad0e-fcc3-4aa7-b735-9e4c597cb4a7.png)
