[test] added test for ToTensor transform on grayscale images

2 jobs for to-tensor-gray in 12 minutes and 52 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #156192
docker
build_linux_36

00:10:39

71.0%
passed #156193
macosx
build_macosx_36

00:02:13