[caffe2] Disable running full grad check in tests by default
Summary:
We've been seeing a lot of Hypothesis timeouts and from profiling a few of the failing tests one of the contributing factors is really slow grad checker. In short, it launches the whole op for each of the input elements so the overall complexity is O(numel^2) at least.
This applies a very unscientific hack to just run grad check on the first and last few elements. It's not ideal, but it's better than flaky tests. One can still explicitly opt in with the env var.
Reviewed By: malfet
Differential Revision: D23336220
fbshipit-source-id: f04d8d43c6aa1590c2f3e72fc7ccc6aa674e49d2
