Add run_gsg.py and run_gsg_no_trainer.py pre-training scripts to examples
### Feature request

Models such as Pegasus and LongT5 have been pretrained using Gap Sentences Generation (GSG) strategy rather the typical Masked Language Modelling (MLM). 

This pre-training strategy leads to improved performance in certain language tasks such as [summarisation](https://arxiv.org/pdf/1912.08777.pdf). This request is to add run_gsg_.py and run_gsg_no_trainer.py files in the examples folder that would enable pre-training using the GSG strategy instead of or on top of MLM.

### Motivation

This will enable users to pre-train Pegasus or LongT5 models from scratch or to continue pre-training existing checkpoints on new datasets.

### Your contribution

I've started thinking about how to build this and am happy to contribute a PR if the HF team think this is valuable and can offer advice on the best ways to approach this.
We try to avoid having examples that are too specific in the maintained examples, as we don't have the bandwidth for too many of them. How about you host it in a repo of yourself and then link to it from the model pages in our doc as well as the community page?<|||||>Ah I see, not problem that seems like a good alternative. Where would be the best place for asking for help with road blocks if I stumble across any?<|||||>You can use this issue or the [forums](https://discuss.huggingface.co/) :-)<|||||>Thank you @sgugger. I've had to de-prioritise this for due to funding constraints that will delay when we can train a bigger version of LongT5 from scratch so I'll close this for now and if we pick this back up I'll post any qs in the forums.