Added JSON manifest's support to transcribe_speech.py
This pull request **introduces 1 alert** when merging 6ffaa42f691b38c7012ee5ff3c8c6efd538c4546 into c8131785cb8980ddb76c088eb32baf91c3c32495 - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-2551329ce0f2e7e1a90f3b651cfa199021015c86)

**new alerts:**

* 1 for Unused import
Extended `transcribe_speech.py`
- added support for JSON dataset manifest as input (to transcribe datasets for analysis in SDE)
- modified output format to JSON manifest (to store filenames with ASR transcripts)

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