[tests] add the missing require_torch_multi_gpu flag

[faaany]

What does this PR do?

For Non-GPU environment, test_multi_gpu_data_parallel_forward should be skipped instead of failing.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

@ArthurZucker and @amyeroberts

[amyeroberts]

Thanks!