Update torch.ops.higher_order.cond (#247)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/247

In D48769108, the core cond operator was moved to
torch.ops.higher_order.cond, making control_flow.cond contain only the wrapper
for cond, and torch.ops.higher_order.cond is the actual operator in the graph.

Reviewed By: ydwu4

Differential Revision: D49077093

fbshipit-source-id: 1afcbfa82e0f660e22bc151173ba76f4b9c78eb0