NeMo Distillation API
@pablo-garay @ko3n1g  I'd like to request a review, thanks! @pablo-garay is OOTO til Aug 24.

@ko3n1g You're a code owner for same file, so it's redundant.  Could we bypass Pablo so we can merge? hey @AAnoosheh, I'm still working on a fix for the CI. Unless it's highly business critical I would prefer waiting until our infra is up again such that we fully test this PR @ChenhanYu How business critical is this atm? Hi I want to use this code for Distillation.
I was not able to find **hide_teacher_model** function in "examples/nlp/language_modeling/megatron_gpt_distillation.py" and I got error.

Where can I find **hide_teacher_model** function?
 Hi, @sangHa0411 

The `hide_teacher_model` is part of the Distillation module in the [Model Optimizer library](https://github.com/NVIDIA/TensorRT-Model-Optimizer) but not officially publicized yet. 

If you have v0.15 of the library installed and use the script as is, it should be present in the model after it gets converted – on [this line](https://github.com/NVIDIA/NeMo/blob/d7d6b99c22d3edaeb0f61b2b1719e4e400f540af/examples/nlp/language_modeling/megatron_gpt_distillation.py#L160) “Pipeline parallelism not yet supported”，then how to deal with the models like llama 70B? Do we apply the tensor parallelism 
 across multiple nodes? @lyravv
Llama-70b can't even be tensor-parallel split further than 8 devices as it has GQA with 8 groups.
It would require shard duplication, and I'm not sure if NeMo already supports that or not.

But PP support is underway and hopefully available soon. Now the TP has much less TFLPs than PP. So PP support is very important and I am very interest in it. By the way, how to print the `ce_loss` and `kd_loss` before combine them together? I tried to add `print` in /usr/local/lib/python3.10/dist-packages/modelopt/torch/distill/loss_balancers.py at `StaticLossBalancer::forward`, but it seems the logging is disabled and no output. Does anyone has similar issue? Thanks.
# What does this PR do ?

Add Logits-only Distillation ability to NeMo GPT models.

**Collection**: NLP

# Changelog 
- Full-fledged example script containing a distillation-enabled subclass of `MegatronGPTModel` and proper loss function for logit-based knowledge distillation.
- Default config file, docs, and CICD test

# Usage

```bash
STUDENT_CKPT=".../model1.nemo"  # can also be None
TEACHER_CKPT=".../model2.nemo"
TOKENIZER=".../tokenizer.model"
FINAL_SAVE_FILE="final_checkpoint.nemo"
TP=4
PP=1
NPROC=$((TP * PP))
launch_config="torchrun --nproc_per_node=$NPROC"

${launch_config} examples/nlp/language_modeling/megatron_gpt_distillation.py \
    model.restore_from_path=$STUDENT_CKPT \
    model.kd_teacher_restore_from_path=$TEACHER_CKPT \
    model.tensor_model_parallel_size=${TP} \
    model.pipeline_model_parallel_size=${PP} \
    model.micro_batch_size=1 \
    model.tokenizer.model=$TOKENIZER \
    model.nemo_path=$FINAL_SAVE_FILE \
    trainer.precision=bf16 \
    trainer.devices=$NPROC
```

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# Additional Information
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