Update CI workflow to ignore tests which require GPU

[jeipollack]

Validation tests for training and metrics evaluation are launched with pytest. However, because they require the usage of GPU they're not suitable to run during CI because GPUs available on Github-hosted runners are scarcely-available.

In release-2.0.0., the validation tests are skipped with the pytest.mark.skip decorator. Instead we can use the decorator pytest.mark.skipif, adding a conditional to be evaluated that will skip the test only during CI.