[testing] rename skip targets + docs
As discussed in https://github.com/huggingface/transformers/issues/6349 this PR brings consistency to skip decorators, so now we will have:

* `require_torch` - this test will run only under torch
* `require_torch_gpu` - as `require_torch` plus requires at least 1 GPU
* `require_torch_multigpu` - as `require_torch` plus requires at least 2 GPUs
* `require_torch_non_multigpu` - as `require_torch` plus requires 0 or 1 GPUs
* `require_torch_tpu` - as `require_torch` plus requires at least 1 TPU

Documentation updated and expanded.

The main change was done by running:
```
find . -type d -name ".git" -prune -o -type f -exec perl -pi -e 's|require_multigpu|require_torch_multigpu|g' {} \;
find . -type d -name ".git" -prune -o -type f -exec perl -pi -e 's|require_torch_and_cuda|require_torch_gpu|g' {} \;
find . -type d -name ".git" -prune -o -type f -exec perl -pi -e 's|require_non_multigpu|require_torch_non_multigpu|g' {} \;
```

Fixes: #6349

@LysandreJik, @sgugger, @sshleifer 
I resolved conflicts - it's good to go now.