message: Build fix for missing precision_config.
  THe conversion from dot_general to dot fails when trying to retrieve
  and use the precision config, since precision_config is optional.
diff: @@ -167,8 +167,10 @@ struct GeneralDotConvert : public OpRewritePattern<mlir::mhlo::DotGeneralOp> {
  auto new_dot_type =
  RankedTensorType::get({lhs_shape[0], rhs_shape[1]}, dot_element_type);
  + mlir::ArrayAttr precision_config;
  + if (op.precision_config()) precision_config = *op.precision_config();
  auto new_dot_op = rewriter.create<mlir::mhlo::DotOp>(
  - op.getLoc(), new_dot_type, lhs, rhs, *(op.precision_config()));
  + op.getLoc(), new_dot_type, lhs, rhs, precision_config);
  rewriter.replaceOpWithNewOp<mlir::mhlo::ReshapeOp>(op, op.getType(),
  new_dot_op);
  