Added new features to Speech Data Explorer
This pull request **introduces 3 alerts** when merging 4d597442a3580a54ead14a7343604b4261978649 into 14904fd0381f5515a9773cf7a538dc09e08c8aa9 - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-cc8577ccccee49e7cd143e645da650effcd2095f)

**new alerts:**

* 2 for Unused import
* 1 for Variable defined multiple times
- support for errors' analysis (WER, CER, Word Matching Rate, word accuracy, diff)
- caching of computed metrics, statistics
- UI updates (hideable columns)

Signed-off-by: Vitaly Lavrukhin <vlavrukhin@nvidia.com>