Avoid copy when returning python numpy / array

[axsaucedo]

PyBind11 allows for efficient data passing between C++ and python, particularly in cases where the memory is owned by the C++ program, and the data is passed upstream to Python. This can be implemented in a simple way via either the def_buffer definition, or alternatively by casting the data into a py::capsule which seems to be a non-owning memory container. References:

• Documentation containing info about def_buffer https://pybind11.readthedocs.io/en/stable/advanced/pycpp/numpy.html
• Issue discussing how to use py::capsule

This issue would depend on whether part 4 of #136 is carried out, as removing the std::vector completely, would result in having just a std::span which is a non-owning "memory-view" to the host-visible GPU memory, and there may be hidden nuances when passing this non-owning view to python as a buffer (including non-explicit performance nuances that may be better to make explicit, such as by recommending to minimise copies to/from tensors).

[axsaucedo]

This was done via #164