[testing] rename skip targets + docs

[stas00]

As discussed in #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

[sgugger]

Thanks a lot for doing this! Just added some nits.

[stas00]

I resolved conflicts - it's good to go now.

[LysandreJik]

Very welcome change! Thanks @stas00!