Decoder model [Duplex Text Normalization] train failing, using TarredDataset
**Unable to start training Decoder** model **using tarred dataset** obtained from script [create_tarred_dataset.py](https://github.com/NVIDIA/NeMo/blob/main/examples/nlp/duplex_text_normalization/data/create_tarred_dataset.py).

The error occurs on the line:
https://github.com/NVIDIA/NeMo/blob/dd921e792427e3788ad9fe3a1e568afc85a8bc93/nemo/collections/nlp/data/text_normalization/decoder_dataset.py#L537

and looks like:

`ValueError: didn't find ['pkl'] in ['__key__', 'tsv-batch-3649.pkl', 'tsv-batch-3650.pkl', 'tsv-batch-3651.pkl', 'tsv-batch-3652.pkl', ....]` (where by "...."  I mean enumeration of other .pkl files).

As I understand it, it would be more correct to do the following in [decoder_dataset.py](https://github.com/NVIDIA/NeMo/blob/main/nemo/collections/nlp/data/text_normalization/decoder_dataset.py#L537):

1. Obtain every filename with ".pkl" extension and create `pkls` (List). After, create a string like:
```
pkls = ';'.join(pkls)
```

2. Rename as follows (using every key, separated by ";" - **supports by WebDataset**)
```
self._dataset = self._dataset.rename(pkl=pkls, key='__key__').to_tuple('pkl', 'key').map(f=self._build_sample)
```

However, this method also does not allow you to start training.