text_generation_utils memory reduction if no logprob needed
# What does this PR do ?

make text_generation_utils.py more memory efficient for inference especially for long context sequences 
* by making logprob computation optional

**Collection**: nlp

# Changelog 
- change peft eval model class for restoring to MegatronGPTSFTModel, since NLPModel does not have _build_model
- removing logprob computation in text_generation_utils since this is very memory consuming because output embedding are all gathered for this. reusing parameter compute_logprob to check if this is needed, by default false

# Usage

```python
 python examples/nlp/language_modeling/megatron_gpt_eval.py \
    ++trainer.devices=${DEVICES} \
    gpt_model_file=$CKPT \
    trainer.num_nodes=1 \
    inference.greedy=True \
    pipeline_model_parallel_size=$PP \
    tensor_model_parallel_size=$TP \
    trainer.precision=bf16 \
    inference.compute_logprob=True \

python examples/nlp/language_modeling/tuning/megatron_gpt_peft_eval.py \
 inference.compute_logprob=True ..
```

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**PR Type**:
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- [ ] Bugfix
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# Additional Information
* Related to # (issue)
