Fix for old models in change_attention_model
# What does this PR do ?

Fixes some issues:
-change_attention_model should work for old models which don't have some of the relevant parameters in config
-also it shouldn't run when nothing to change was submitted

Original PR: https://github.com/NVIDIA/NeMo/pull/6608

**Collection**: ASR

  
**PR Type**:
- [ ] New Feature
- [X] Bugfix
- [ ] Documentation
