Support faiss_index_gpu_to_cpu in GpuIndexImpl

[Alexander A.]

Now GpuIndexImpl supports only cpu -> gpu

Add support function faiss_index_gpu_to_cpu

[Eduardo Pinho]

I'm not sure I follow. Don't GpuIndexImpl#to_cpu and GpuIndexImpl#into_cpu work for converting a GPU index back to a CPU index? source

[Alexander A.]

Oh! I am sorry!
I did not see