Fix to peft & virtual pipeline parallel unsupported check
@cuichenx / @ericharper can you review?  Thanks! jenkins
# What does this PR do ?

Corrects the check added in [PR-7964](https://github.com/NVIDIA/NeMo/pull/7964) to raise a ValueError when attempting to run virtual pipeline parallel with peft.

**Collection**: nlp/language_modeling

# Changelog 
- Moves this safety check from `MegatronGPTSFTModel.__init__` (where self.use_peft had not yet been set to true, so the check did not function) to `NLPAdapterModelMixin.add_adapter`

# Usage
Running with the unsupported combination of peft and virtual pipeline parallel now yields a more useful error message:

```
python examples/nlp/language_modeling/tuning/megatron_gpt_peft_tuning.py \
    ++model.virtual_pipeline_model_parallel_size=10 \
    ++model.peft.peft_scheme=lora \
    ...
```
gives
```
  File "/opt/NeMo/examples/nlp/language_modeling/tuning/megatron_gpt_peft_tuning.py", line 73, in main
    model.add_adapter(peft_cfg_cls(model_cfg))
  File "/opt/NeMo/nemo/collections/nlp/parts/mixins/nlp_adapter_mixins.py", line 178, in add_adapter
    raise ValueError('Virtual pipeline model parallel is not supported when using PEFT')
```

# Jenkins CI
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