fix swig ownership when assigning to struct pointer field

[mdouze]

Summary: When faiss is called from SWIG, it is usually assumed that Python will do all the memory management, therefore assigning to a pointer in C++ should not transfer ownerhsip (default SWIG behavior). This diff fixed that.

Differential Revision: D51529085

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D51529085

[mdouze]

hopefully this will finally fix https://github.com/facebookresearch/faiss/issues/2996

[junjieqi]

@mdouze do we still need this one?

[mnorris11]

Even though the final assert fails since the ownership disappears, it seems like the memory does not increase. This typemap looks right to me when I test it. We can discuss on next steps.

[mnorris11]

Current status after discussion:

ASAN does not seem to detect these Python memory leaks. There are no ASAN errors in a simple repro unit test like

def test_ownership_2(self):
        d = 32
        quantizer = faiss.IndexFlatL2(d)
        quantizer.this.own(False)

The destructor of IndexFlatL2 does not get called when quantizer.this.own(False). Removing this line will call the destructor.

We should introduce some Python unit test to catch these memory leaks.