message: Improve the shape function of Bincount
  There was not a lot of restriction in shape function
  of Bincount and the output shape was unknown.
  It is actually possible to get a better shape output
  if `size` input is known.
  This fix adds enhancement to the shape function of
  Bincount.
diff: @@ -1416,6 +1416,10 @@ REGISTER_OP("Bincount")
  .Attr("T: {int32, int64, float32, float64}")
  .Output("bins: T")
  .SetShapeFn([](InferenceContext* c) {
  + ShapeHandle unused;
  + // The input `size` must be a scalar.
  + TF_RETURN_IF_ERROR(c->WithRank(c->input(1), 0, &unused));
  +
  c->set_output(0, c->UnknownShapeOfRank(1));
  return Status::OK();
  });
  