Fix setting up of `ReduceLROnPlateau` learning rate scheduler
# What does this PR do ?

Fix setting up of `ReduceLROnPlateau` learning rate scheduler.

**Collection**: core

Steps to reproduce
```bash
git clone https://github.com/NVIDIA/NeMo.git
cd NeMo
git checkout fix/reduce_on_plateau_r1.11.0
git checkout r1.11.0 -- nemo/core/optim/lr_scheduler.py nemo/core/config/schedulers.py
pytest tests/core/test_optimizers_schedulers.py
```


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**Pre checks**:
- [ ] 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?
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  - [ ] Reviewer: Does the PR have correct import guards for all optional libraries?
  
**PR Type**:
- [ ] New Feature
- [x] Bugfix
- [ ] Documentation

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# Additional Information
* Related to # (issue)
