message: [XLA] Erase cloned instructions on the fly when merging fusion nodes.
  This avoids the awkward situation where an RNG which is clearly eligible for fusion becomes ineligible mid-fusion because it suddenly has an extra (dead) user.
diff: @@ -716,10 +716,12 @@ void HloInstruction::MergeFusionInstructionIntoMultiOutput(
  // Fuse the root instruction and generate multiple outputs.
  FuseInstructionIntoMultiOutput(unfused_root);
  + TF_CHECK_OK(unfused_root->parent()->RemoveInstruction(unfused_root));
  // The rest instructions are of normal fusing.
  for (int64 i = 1; i < unfused_instructions.size(); i++) {
  auto instruction = unfused_instructions[i];
  FuseInstruction(instruction);
  + TF_CHECK_OK(instruction->parent()->RemoveInstruction(instruction));
  }
  }
  