Evaluation script for VAD models
**Is your feature request related to a problem? Please describe.**

I'd like to be able to evaluate the performance of speech/audio classification models (in particular, VAD/SAD models) with different collars, using metrics such as are available in `md-eval.pl` in SCTK (used by Kaldi's SAD recipes) or PyAnnote metrics (see https://github.com/pyannote/pyannote-metrics/blob/develop/pyannote/metrics/detection.py).

**Describe the solution you'd like**

I'd like to have a script that takes a manifest (formatted the same way as for train/validation in speech classification; not necessarily preprocessed to have only short sections) as input and produces error/accuracy metrics. As PyAnnote Metrics is already used elsewhere in NeMo, it could probably easily used for this purpose. Or perhaps there already exist internal/unpublished scripts for that purpose that could be added to NeMo.

**Describe alternatives you've considered**

Creating RTTM output from VAD predictions and evaluating them using SCTK/md-eval.pl.

Also tagging @fayejf

Thanks!