Disable TestGroupNormBackward on CPU DeviceType (#2313)
The previous tensorflow update (62eb0b1) appears to have caused a 30+ minute runtime regression on the CPU test for AtenXlaTensorTest.TestGroupNormBackward. Because a forward fix is unknown at this time, disable the test for CPU devices in CircleCI.
Test plan: check that the runtime of pytorch_xla_linux_bionic_py3_6_clang9_CPU_test_and_push_doc does not exceed 1.5 hours.
