cpp API Adam optimizer test is flaky

```
16:00:21 /private/var/lib/jenkins/workspace/pytorch-builds/pytorch-macos-10.13-py3-build-test/test/cpp/api/optim.cpp:93: FAILED:
16:00:21   REQUIRE( test_optimizer_xor(optim, model) )
16:00:21 with expansion:
16:00:21   false
```

https://ci.pytorch.org/jenkins/job/pytorch-builds/job/pytorch-macos-10.13-py3-build-test/6074//console

CC @ebetica @goldsborough 
