[Bugfix] Move nemo file config logic of GPT eval script
# What does this PR do ?

Move the cfg.gpt_model_file logic into the "if" block to avoid error during ckpt eval

**Collection**: [Note which collection this PR will affect] 
NLP
examples/nlp/language_modelling

# Changelog 
- Add specific line by line info of high level changes in this PR.
Move https://github.com/NVIDIA/NeMo/blob/main/examples/nlp/language_modeling/megatron_gpt_eval.py#L172-L189
to after https://github.com/NVIDIA/NeMo/blob/main/examples/nlp/language_modeling/megatron_gpt_eval.py#L191

# Usage
* You can potentially add a usage example below
Otherwise, if following this
```python
        python megatron_gpt_eval.py \
            checkpoint_dir=PATH_TO_CHECKPOINT_FILE \
            checkpoint_name=CHECKPOINT_FILE_NAME \
            hparams_file=HPARAMS_FILE \
            inference.greedy=True \
            inference.add_BOS=True \
            trainer.devices=1 \
            trainer.num_nodes=1 \
            tensor_model_parallel_size=-1 \
            pipeline_model_parallel_size=-1 \
            prompts=[prompt1,prompt2]
```
Since in this case gpt_model_file=null, the current script will error out due to restore path unknown
```
Traceback (most recent call last):
  File "megatron_gpt_eval.py", line 176, in main
    model_config = MegatronGPTModel.restore_from(
  File "/workspace/NeMo-1.18.0/nemo/core/classes/modelPT.py", line 425, in restore_from
    restore_path = os.path.abspath(os.path.expanduser(restore_path))
  File "/usr/lib/python3.8/posixpath.py", line 231, in expanduser
    path = os.fspath(path)
TypeError: expected str, bytes or os.PathLike object, not NoneType

```


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**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? No
- [ ] Did you add or update any necessary documentation? No
- [ ] Does the PR affect components that are optional to install? (Ex: Numba, Pynini, Apex etc) No
- [ ] Reviewer: Does the PR have correct import guards for all optional libraries?
  
**PR Type**:
- [ ] New Feature
- [x] Bugfix
- [ ] Documentation

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## Who can review?

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[Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md) contains specific people who can review PRs to various areas.

# Additional Information
* Related to # (issue)
