Fix import error of megatron.core in tests
# What does this PR do ?

After the standard installation of NeMo, running `pytest tests/` in code repo will report the error:
```
______________________________________________________________________________________ ERROR collecting tests/collections/nlp/test_flash_attention.py ______________________________________________________________________________________
ImportError while importing test module '/home/robin/try1/NeMo/tests/collections/nlp/test_flash_attention.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
../../miniconda3/envs/nemo/lib/python3.10/importlib/__init__.py:126: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/collections/nlp/test_flash_attention.py:19: in <module>
    from megatron.core import ModelParallelConfig
E   ImportError: cannot import name 'ModelParallelConfig' from 'megatron.core' (/home/robin/miniconda3/envs/nemo/lib/python3.10/site-packages/megatron/core/__init__.py)
```

The `requirements/requirements_nlp.txt` fixed version of `megatron_core` to `0.2.0`. But in this version, the Megatron-LM doesn't even have `ModelParallelConfig`.
The fix is learning from `nemo/collections/nlp/models/language_modeling/megatron/gpt_model.py`. By using `ApexGuardDefaults` to replace `ModelParallelConfig`, the `pytest tests/collections/nlp/test_flash_attention.py` could pass without import error.

**Collection**: tests

# Changelog 
- Fix import error of megatron.core in tests

# 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?
- [ ] 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?

@MaximumEntropy @ericharper @ekmb @yzhang123 @VahidooX @vladgets @okuchaiev @XuesongYang 
