skip test_trace_c10_ops if _caffe2 is not built (#19099)
Summary:
fix https://github.com/pytorch/pytorch/issues/18142
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19099
Differential Revision: D15010452
Pulled By: houseroad
fbshipit-source-id: 5bf158d7fce7bfde109d364a3a9c85b83761fffb
