[ASR] Add pretrained ASR models for Croatian
Signed-off-by: Ante Jukić <ajukic@nvidia.com>

# What does this PR do ?

This PR adds pre-trained models for Croatian.

**Collection**: [ASR]

# Changelog 
- updated `PretrainedModelInfo` and docs
- minor (cosmetic) modifications to `checkpoint_averaging.py`

# Usage
Tested with the following

```python
from nemo.collections.asr.models import ASRModel

for type in ['ctc', 'transducer']:
    model = ASRModel.from_pretrained(f'stt_hr_conformer_{type}_large', map_location='cpu')

    print(f'\nmodel_{type}.summarize()')
    print(model.summarize())
    print()
```

Pretrained models downloaded from NGC.

# 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

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## 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.

