[NLP] Access scaler only in FP16 case
# What does this PR do ?

When trying to load FP16 model with `trainer.precision != 16` (or `16-mixed`) a user gets a mysterious error message like:
```
  File "/opt/NeMo/nemo/collections/nlp/models/language_modeling/megatron_base_model.py", line 122, in __init__
    self.model_parallel_config: ModelParallelConfig = self.build_model_parallel_config()
  File "/opt/NeMo/nemo/collections/nlp/models/language_modeling/megatron_base_model.py", line 782, in build_model_parallel_config
    "grad_scale_func": self.trainer.precision_plugin.scaler.scale
AttributeError: 'NoneType' object has no attribute 'scale'
```
I'm changing the code to access `trainer.precision_plugin.scaler.scale` only when `trainer` is configured with the relevant precision: "16" or "16-mixed".

**Collection**: NLP

# Before your PR is "Ready for review"
**Pre checks**:
- [x] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
- [ ] Did you write any new necessary tests?
- [x] 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
- [x] Documentation

If you haven't finished some of the above items you can still open "Draft" PR.


## Who can review?

Anyone in the community is free to review the PR once the checks have passed. 
[Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md) contains specific people who can review PRs to various areas.