using multi_gpu consistently
As discussed [here](https://github.com/huggingface/transformers/pull/8341#issuecomment-722705833) this PR replaces 
* `multiple_gpu`
* `multigpu`
with `multi_gpu` for consistency

There is no functionality change otherwise.

I did repo-wide:
```
find . -type d -name ".git" -prune -o -type f -exec perl -pi -e 's#(multiple_gpu|multigpu)#multi_gpu#g' {} \;
```

@LysandreJik, @sgugger 