Fix setting up of `ReduceLROnPlateau` learning rate scheduler

[PeganovAnton]

What does this PR do ?

Fix setting up of ReduceLROnPlateau learning rate scheduler.

Collection: core

Steps to reproduce

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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• [ ] New Feature
• [x] Bugfix
• [ ] Documentation

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Additional Information

• Related to # (issue)