Update diarization data loader to train meeting data
Signed-off-by: Taejin Park <tango4j@gmail.com>

# What does this PR do ?

This PR adds a new feature to `audio_to_diar_label.py`.
So far, we have only trained with 2 speaker audio where one of the speakers always exists.
This PR makes dataloader to take 2 speaker subset from more-than-2-speaker audio clips with corresponding groundtruth.
Thus, there are cases where grround-truth-cluster label is "-1" where speakers are not included in either of two targeted speakers.
ex) Two speaker ground-truth label ([0,0,1,1,-1,-1,-1,1,1,1,0]

Also removed unnecessary variables.

**Collection**: [Note which collection this PR will affect]

ASR

# Changelog 

-  `audio_to_diar_label.py`

-- Removed unused variables in the functions.  
-- base_clus_label now can have -1 label. 
-- Few cosmetic changes

-  `collections.py`

A few changes include:

-- Added get_rttm_speaker_index() function since sess_spk_dict could have more than 2 speakers.

-  `speaker_utils.py`

-- Added get_rttm_speaker_index that takes speakers from RTTM files.

# Usage

The usage will be provided when model and module codes are published.

# Before your PR is "Ready for review"
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**PR Type**:
- [x] New Feature
- [ ] Bugfix
- [ ] Documentation

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# Additional Information
* Related to # (issue)
