Thank you for the fast answer:
My bad, I’m currently using the 1.3.1 version.
Using pin_memory=False results to a slightly longer time per epoch ( approximately 3 to 5 sec) but I don’t know if it is consequential.
I’ll try to rerun my code with the nighty build.
Edit: with the nighty build (‘1.4.0.dev20191116’), I have approximately the same time.
I also monitored the gpu usage and memory and there is approximately 800 more Mo used  when
net.to(device=gpu) is executed. So it seems GPU is actually used.