Update tarred dataset conversion script
This pull request **introduces 6 alerts** when merging 57bbdd1984e6529cb18f138488bad775e10a19b1 into c0fedd91e316a4ba95ac6b14330ff92844c56097 - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-fc89a4569b49ec7de84b75ce25de8500ede41b9d)

**new alerts:**

* 4 for Unused import
* 2 for Syntax error
Bugfix for if maximum duration set to None
Added minimum duration filter