message: Update to latest poplar API for non-linearities
  Summary: The namespace for non-linearityies has changed
  Test Plan: existing tests building
  Reviewers: georgep
diff: @@ -469,7 +469,8 @@ StatusOr<poplar::program::Program> CreateReluGradOp(
  poplar::program::Sequence seq;
  poplar::Tensor t = popnn::nonLinearityInputGradient(
  - graph, popnn::NON_LINEARITY_RELU, out, outgrad, seq, GetDebugName(inst));
  + graph, popnn::NonLinearityType::RELU, out, outgrad, seq,
  + GetDebugName(inst));
  TF_ASSIGN_OR_RETURN(t, BroadcastTensor(t, output_shape));
  @@ -508,7 +509,8 @@ StatusOr<poplar::program::Program> CreateSigmoidGradOp(
  poplar::program::Sequence seq;
  poplar::Tensor t =
  - popnn::nonLinearityInputGradient(graph, popnn::NON_LINEARITY_SIGMOID, out,
  + popnn::nonLinearityInputGradient(graph,
  + popnn::NonLinearityType::SIGMOID, out,
  outgrad, seq, GetDebugName(inst));
  TF_ASSIGN_OR_RETURN(t, BroadcastTensor(t, output_shape));
  