Don't overwrite gravity _if_ specified in yaml config file

[sheim]

Currently, whatever the user writes into the config file for gravity is overwritten in vec_task.py, which is both confusing and not useful. Now checks if the cfg specifies gravity, and only writes default values if it doesn't.

[kellyguo11]

Hi there, it appears that some of our tasks are making a call to set_sim_params_up_axis(), which is overwriting the gravity values that are specified in the config files. We are thinking of removing the call to set_sim_params_up_axis() from those tasks to avoid the confusion. With this change, we would be reading both up_axis and gravity values directly from the config files. Would that work for your use case?

[sheim]

I suppose that would, but I've also switched to using isaacGym3 together with legged_gym anyway, so feel free to do what works best for you.