prefix tuning for Megatron gpt models
# What does this PR do ?

implements prefix tuning for Megatron GPT models.

**Collection**: `examples/nlp/languge_modeling`

# Changelog 
- Add specific line by line info of high level changes in this PR.

# Usage
```
/NeMo/examples/nlp/language_modeling$python megatron_gpt_prefix_tuning.py
```
The prefix tuning `.yaml` file is here: `/NeMo/examples/nlp/language_modeling/conf/megatron_gpt_prefix_tuning_config.yaml`, the settings in this config are used by default when `NeMo/examples/nlp/language_modeling/megatron_gpt_prefix_tuning.py` is launched.

Once trained, you can evaluate using:
```
python megatron_gpt_eval.py \
    prefix_tuned_model_file=<PATH_.nemo_FILE_GENERATED_AT_END_OF_TRAINING> \
    gpt_model_file=<PATH_TO_.nemo_FILE_WITH_PRETRAINED_GPT_MODEL> \
    inference.greedy=True \
    inference.add_BOS=False \
    trainer.devices=1 \
    trainer.num_nodes=1 \
    tensor_model_parallel_size=1 \
    pipeline_model_parallel_size=1 \
    prompts=["<PATH_TO_.jsonl_FILE_TO_EVALUATE>"]
```

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**Pre checks**:
- [ ] 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**:
- [x] New Feature
- [ ] Bugfix
- [ ] 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. 
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# Additional Information
* Related to # (issue)
