Virtual pipeline parallel support for MegatronGPTSFTModel
This PR is stale because it has been open for 14 days with no activity. Remove stale label or comment or update or this will be closed in 7 days. jenkins jenkins
# What does this PR do ?

Enables running MegatronGPTSFTModel with virtual pipeline parallel when peft_scheme=null.

**Collection**: NLP (language_modeling)

# Changelog 
- Alters word embedding initialization and passing of several parameters in MegatronGPTSFTModel to accommodate virtual pipeline parallelism when self.use_peft is false
- Adds a guard in `MegatronGPTSFTModel.__init__` to raise a ValueError if self.use_peft is true
- Corrects the TP group initialization check in MegatronGPTSFTModel to account for both TE and MCore flags

# Usage
Run a GPT model with SFT and set `virtual_pipeline_model_parallel_size`, such as:

```
python examples/nlp/language_modeling/tuning/megatron_gpt_peft_tuning.py \
++model.virtual_pipeline_model_parallel_size=10 \
...
```

# Jenkins CI
To run Jenkins, a NeMo User with write access must comment `jenkins` on the PR.

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