Add nvidia-ml-py as pip dependency

[rsmallegoor]

Adds nvidia-ml-py which can be used to retrieve information from an NVIDIA GPU.

Required by this PR which add a GPU monitor to the diagnostic tools.

[mikaelarguedas]

Thanks @rsmallegoor for the links and the context.

Can you please add a debian rule for this package as well?

Note that any package depending on pip keys (or any non apt key) cannot be released as a debian package. So if the linked PR gets accepted, diagnostic_common_diagnostics cannot be released in ROS anymore

[rsmallegoor]

@mikaelarguedas Based on your comments and this discussion, I've decided to create a standalone package.