Confidence ensembles: fix issues and add tuning functionality
# What does this PR do ?

Adding new functionality for confidence ensembles and fix existing issues. Note that there is a plan with another follow-up PR to fix (partially) existing TODOs in the code + add docs, types and a tutorial on the usage.

**Collection**: ASR

# Changelog 
- In the previous PR I directly used frame confidence returned from NeMo. This is problematic for two reasons. First, it does not filter blanks, even when exclude_blanks is set to True in the config. Second, it can only compute a single confidence, so there is a problem with using it in a tuning mode where we want to get 100s of confidence values at the same time. So to overcome these problems, I wrote `compute_confidence` function, alongside with a few additional utilities that does what we need for confidence ensembles outside of the decoding code.
- Implemented tuning for logistic regression parameters as well as for the confidence parameters. This requires user to specify dev dataset and run transcribe over it.
- Added more tests for tuning of the confidence.

# Usage
* You can potentially add a usage example below

```python
# Add a code snippet demonstrating how to use this 
```

# 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)
- [x] Did you write any new necessary tests?
- [ ] 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
- [x] Bugfix
- [ ] Documentation

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


## Who can review?

Anyone in the community is free to review the PR once the checks have passed. 
[Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md) contains specific people who can review PRs to various areas.

# Additional Information
* Related to # (issue)
