Remove incorrect extra argument of load_from_checkpoint_dir()
# What does this PR do ?

The function load_from_checkpoint_dir() in `examples/nlp/language_modeling/megatron_gpt_continue_training.py` has four arguments:
```python
def load_from_checkpoint_dir(cls, cfg, trainer, modify_confg_fn):
```
But the calling side is:
```python
model = load_from_checkpoint_dir(MegatronGPTModel, cfg, trainer, gpt_cfg, modify_confg_fn=_modify_config)
```
The argument `gpt_cfg` is extraneous.

# Changelog 
- Remove extraneous argument of load_from_checkpoint_dir()

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- [ ] 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

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## Who can review?

@MaximumEntropy @ericharper @ekmb @yzhang123 @VahidooX @vladgets @titu1994 @XuesongYang 

# Additional Information
* Related to #7480 
