stop dealloc of coarse quantizer when it is deleted (#2603)

Summary:
Pull Request resolved: #2603

X-link: facebookresearch/faiss#4045

Need to add to `__init__.py` like Matthijs mentioned on the github issue facebookresearch/faiss#3993. But we can't do it for non-GPU code, otherwise it will throw an exception and fail many tests than include fbcode/faiss. So we need to check if FAISS GPU is importable first.

To find the class names like GpuIndexIVFFlat etc, I checked everything under faiss/gpu where the constructor accepts an Index. The other Index is always parameter at index 1 (0-indexed), so that's why we use 1 in the function calls.

Reviewed By: pankajsingh88

Differential Revision: D66675910

fbshipit-source-id: f170dadb6318c620420689164f9522f9815aa980

examples/retrieval/knn_index.py

@@ -43,7 +43,6 @@ def get_index(
         res = faiss.StandardGpuResources()
         # pyre-fixme[16]
         config = faiss.GpuIndexIVFPQConfig()
-        # pyre-ignore[16]
         index = faiss.GpuIndexIVFPQ(
             res,
             embedding_dim,