Skip fp16 initializer test for CPU-only builds
Skip fp16 initializer test for CPU-only builds

Summary:
Working towards https://github.com/caffe2/caffe2/pull/817.
```
E           AttributeError: Method FloatToHalf is not a registered operator. Did you mean: []
```
https://travis-ci.org/caffe2/caffe2/jobs/243867951

/cc slayton58
Closes https://github.com/caffe2/caffe2/pull/829

Differential Revision: D5276796

Pulled By: akyrola

fbshipit-source-id: 34edca6090a9ce7ab39ae1fdc0e83b5c3b7e4f49

pietern/caffe2,xzturn/caffe2,Yangqing/caffe2,xzturn/caffe2,pietern/caffe2,xzturn/caffe2,caffe2/caffe2,xzturn/caffe2,Yangqing/caffe2,pietern/caffe2,Yangqing/caffe2,davinwang/caffe2,bwasti/caffe2,sf-wind/caffe2,bwasti/caffe2,bwasti/caffe2,bwasti/caffe2,sf-wind/caffe2,davinwang/caffe2,davinwang/caffe2,sf-wind/caffe2,Yangqing/caffe2,davinwang/caffe2,xzturn/caffe2,sf-wind/caffe2,pietern/caffe2,bwasti/caffe2,sf-wind/caffe2,pietern/caffe2,Yangqing/caffe2,davinwang/caffe2