Run pytorch test only if dependency is installed

[domna]

From openjournals/joss-reviews#8503 (comment)

[...] The test tests/test_TiO2.py::TestTiO2::test_solver4x4_expm_torch failed with an ImportError: PyTorch is not installed. The core test suite should not fail if an optional dependency like PyTorch is not installed. Tests for optional features should be skipped automatically. Please modify this test to use pytest.mark.skipif. [...]