[MPNet] example of fine-tuning MPNet language model on domain specific corpus
# 🚀 Feature request

I'd like to understand if it's possible to fine-tune MPnet model on domain specific corpus. I tried to run following script for MPNet and it seemed to be working (or at least on throwing any errors).
`python run_mlm.py --model_name_or_path microsoft/mpnet-base --dataset_name wikitext --do_train --output_dir tmp/mpnet-output --dataset_config_name wikitext-2-raw-v1`

However, since MPNet combines both MLM and PLM objectives, I'm not clear whether MPNet will actually train properly. 

## Motivation
MPNet establishes SOTA benchmarks on number of tasks. It could be useful to have some examples on how to fine-tune MPNet model on specific corpuses and downstream tasks.

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