Upgrade to PyTorch 1.7

As raised in #36 (closed), we currently depend on PyTorch 1.2, but the latest version is 1.5 1.8.1, while 1.2 is a couple years old.

I think I pinned initially to v1.2 since that's what the original code was working on. Upgrading should go easily though as I don't think there are any major backwards incompatibilities. v1.5 also has some features I would like to use for #17 (it provides some easy to use functions for querying GPU memory usage, which I can use in the tests to have them select a GPU to use based on its current utilization).

Update: Since this issue was opened the latest pytorch version is 1.8.1. I am still leaning towards conservatism on this and selecting a slightly lower version unless there is a 1.8.x feature we specifically need. But PyTorch 1.7 has features for DataLoader that I think will definitely help us, as I explained here: !64 (comment 520852)

I'm close to a point where I can complete this work and hope we can target it for 1.0, but if it takes much longer we can aim it for 1.1.0 too.