Use latest apex internal API
# What does this PR do ?

Apex _fast_layer_norm added mandatory argument `memory_efficient`. This PR fixes call to this function to accommodate to the change.

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# Before your PR is "Ready for review"
**Pre checks**:
- [ ] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
- [ ] 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**:
- [ ] New Feature
- [x ] Bugfix
- [ ] Documentation

## Who can review?

@mikolajblaz can you review this one? I can see that you introduced this line 17 months ago. Can we move to the APEX implementation entirely as suggested in the TODO? 

# Additional Information
* Related to # (issue)
