Configure a ci stage to check for build warnings

This MR adds a pipeline stage to check for warnings in the CPU and CUDA builds.

It does so by saving the build.log output of the build stage in the artifacts and then run a grep in the check-warnings stage.

We could have nicer output messages by saving only the stderr of the build, but ninja merges stdout and stderr (see https://github.com/ninja-build/ninja/issues/1537) so instead I'm using grep which shows only the line containing the warning keyword.

Also important: the check-warnings stage is allowed to fail so it doesn't block the test stages.

This MR revealed a warning in Allen.cpp which I fixed.

@raaij @dovombru @gligorov @cagapopo @sponce @roneil