message: Fix a compile error in dropout.cc in debug tf build
  Reviewers: stephenm
diff: @@ -54,7 +54,8 @@ class DropoutOp : public PoplibsOpDef {
  const HloDropoutInstruction* dropout_instruction =
  dynamic_cast<const HloDropoutInstruction*>(inst);
  - assert(op && "Expected operation to be an xla::poplarplugin::DropoutOp");
  + assert(dropout_instruction &&
  + "Expected operation to be an xla::poplarplugin::DropoutOp");
  // The probabilty that any given element of "x" will be disgarded.
  double rate = dropout_instruction->Rate();
  