[FastPitch Training]  FastPitchDataset AttributeError
**Describe the bug**
I am getting the following error at `FastPitchDataset` while I want to train FP model.
```
  File "/opt/conda/lib/python3.8/site-packages/nemo/collections/asr/data/audio_to_text.py", line 727, in __getitem__
    audio_path = self.collection[item].audio_file
AttributeError: 'FastPitchDataset' object has no attribute 'collection'
```
Can you guys fix it?
Thanks.