This is a near rewrite of our testing configuration. We now use conda to matrix tests against various PyTorch and CUDA versions. The new configuration: runs unit tests with:

1. python=3.6, torch=1.2, cudatoolkit=10.0
2. python=3.7, torch=1.5, cudatoolkit=10.1

The more expensive model tests are only run on the second configuration. We can next extend our configuration to do nightly builds and include torch=1.3 and 1.4 and to also run model tests for all configurations. We should also include cudatoolkit=10.2 soon.