Restore O2 FP16/BF16 wrapper for GPT
Closed by https://github.com/NVIDIA/NeMo/pull/8038.
# What does this PR do ?

When running GPT with O2 enabled, I am seeing errors at the following line since `_wrap_model_for_O2` is not defined:

https://github.com/NVIDIA/NeMo/blob/852338459c78c257a2b5f6688c0b1a5d2b048366/nemo/collections/nlp/models/language_modeling/megatron_gpt_model.py#L266

This function was removed in https://github.com/NVIDIA/NeMo/pull/7814. I see that this PR added the function to BERT, so could this be a bug?

**Collection**: NLP

# Changelog 
- Restore `_wrap_model_for_O2` function in GPT

# Usage

Run GPT, e.g. with the config at https://github.com/NVIDIA/NeMo/blob/main/examples/nlp/language_modeling/conf/megatron_gpt_config.yaml, and set `model.megatron_amp_O2=True`

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Pinging @shanmugamr1992 

# Additional Information
* Bug from https://github.com/NVIDIA/NeMo/pull/7814