figure out a way to load libcudart.so from it's compile-time path

right now if libcudart.so is not in LD_LIBRARY_PATH, though it was found at compile-time, these lines will fail: https://github.com/pytorch/pytorch/blob/master/torch/cuda/__init__.py#L39-L43

Avoid this, by idk doing something...

I think i have a few good ideas.
