fix FLOPsMeasurementCallback on_train_end when experiment is None
@ar-nowaczynski  Can you check if https://github.com/NVIDIA/NeMo/pull/10125 fixes this issue.
# What does this PR do ?

Fixes basic error in recently introduced #9932 FLOPsMeasurementCallback on_train_end method when `pl_module.logger.experiment` is None.

**Collection**: common

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