Add support for Time Stamp calculation using transcribe_speech.py
Signed-off-by: smajumdar <titu1994@gmail.com>

# What does this PR do ?

Adds flag to compute word and char level timestamp using the transcribe speech script.

**Collection**: [ASR]

# Changelog 
- Add specific line by line info of high level changes in this PR.

# Usage

```python

python transcribe_speech.py \
    model_path=null \
    pretrained_name=null \
    audio_dir="<remove or path to folder of audio files>" \
    dataset_manifest="<remove or path to manifest>" \
    output_filename="<remove or specify output filename>" \
    batch_size=32 \
    compute_timestamps=True \
    compute_langs=False \
    cuda=0 \
    amp=True

(Optionally)
    ctc_decoding.ctc_timestamp_type="all"  # (default all, can be [all, char, word])
    rnnt_decoding.rnnt_timestamp_type="all"  # (default all, can be [all, char, word])

```

# Before your PR is "Ready for review"
**Pre checks**:
- [x] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
- [ ] Did you write any new necessary tests?
- [x] Did you add or update any necessary documentation?
- [ ] Does the PR affect components that are optional to install? (Ex: Numba, Pynini, Apex etc)
  - [ ] Reviewer: Does the PR have correct import guards for all optional libraries?
  
**PR Type**:
- [x] New Feature
- [ ] Bugfix
- [ ] Documentation

If you haven't finished some of the above items you can still open "Draft" PR.


## Who can review?
