PP Evaluation for SFT/PEFT
jenkins
# What does this PR do ?

This PR enables the inference of pipeline parallel fine-tuned language models using the `megatron_gpt_generate.py` script. Middle stages during pipeline parallel are expected to have no output during the response generation. We handle this case in the PR to enable evaluation of SFT and PEFT models trained with pipeline parallelism. 

**Collection**: NLP

# Changelog 
- Reorganized logic for generation by checking if generation has any output (line 430).
- Middle stages of pipeline parallel have no output, so we set empty lists on line 439.

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

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