Link with CUPTI to acquire occupancy metrics

[jsjason]

Apparently, CUPTI provides a C++/CUDA interface for retrieving various GPU metrics per kernel. In particular, we are interested in checking the achieved occupancy for a given workload, which is fortunately included in the list of metrics CUPTI covers. Sadly, CUPTI has no Python bindings so we need to manually construct a Python-C bridge to enable/disable CUPTI and fetch the correct metric values (unless we re-write the whole repo in C++... which is probably not going to happen).