message: Force dropout seed output to be {2}.
  Summary:
  Seed is a 2 element tensor of int32s representing
  a int64
  Test Plan: dropout test passes
  Reviewers: georgep, jakeh
diff: @@ -30,6 +30,10 @@ REGISTER_OP("IpuDropout")
  .Attr("scale: float")
  .Attr("is_using_user_seed: bool")
  .Attr("seed_modifier: int")
  - .SetShapeFn(shape_inference::UnchangedShape);
  -
  + .SetShapeFn([](shape_inference::InferenceContext* c) {
  + auto in_shape = c->input(0);
  + c->set_output(0, in_shape);
  + c->set_output(1, c->MakeShape({2}));
  + return Status::OK();
  + });
  } // namespace tensorflow
  