PEFT keeps frozen model in train mode instead of eval mode
jenkins
# What does this PR do ?

We have been experiencing poor convergence when running PEFT in FP8. The root cause is because the frozen model is set in evaluation mode, and so the FP8 scaling factors are never updated from their initial value of 1.0.

**Collection**: NLP

# Changelog 
- PEFT keeps frozen model in train mode instead of eval mode

# Usage

Run GPT finetuning, e.g. with the config at https://github.com/NVIDIA/NeMo/blob/main/examples/nlp/language_modeling/tuning/megatron_gpt_finetuning.py.

Enable FP8 support with model.fp8=True.

# Jenkins CI
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- [x] Bugfix
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# Additional Information
