Errors encountered in tests

[zhengtianze]

Bug Description

I encountered several test failures after a fresh installation of the project. It seems like there are some missing variable definitions and import errors in the test suite.

Error Logs

1. NameError in test_multipole_openmm.py

FAILED tests/test_multipole_openmm.py::test_multipole_against_openmm[False-none-cuda-2-2] - NameError: name 'box' is not defined

2. ImportError in test_lj.py

tests/test_lj.py:6: in <module>
    from torchff.vdw import LennardJones
E   ImportError: cannot import name 'LennardJones' from 'torchff.vdw'

And other errors not listed here.

Any guidance or fixes on this would be greatly appreciated! Let me know if you need more information to debug this.