message: Fix up debug name
  Summary:
  Use the DebugName function to generate a good name, and use the
  input as the reference tensor mapping for the output.
  Test Plan: All tests ok
  Reviewers: georgep, jakeh
  Subscribers: jakeh
diff: @@ -110,8 +110,8 @@ class DropoutOp : public PoplibsOpDef {
  // Perform the actual dropout by calling into the poprand function.
  poplar::Tensor final_output =
  - poprand::dropout(graph, &as_unsgined, seed_modifier, input, reference,
  - rate, scale, seq, "Dropout");
  + poprand::dropout(graph, &as_unsgined, seed_modifier, input, input, rate,
  + scale, seq, GetDebugName(inst));
  // Mark that tensor as our output.
  TF_CHECK_OK(AddOutputTensor(tensor_map, inst, 0, final_output));
  