message: Add default type as DT_FLOAT to maintain backward-compatibility
  and fix test failure in:
  ```
  //tensorflow/core/ops/compat:backwards_compatibility_test
  ```
diff: @@ -688,7 +688,7 @@ REGISTER_OP("NonMaxSuppressionV2")
  .Input("max_output_size: int32")
  .Input("iou_threshold: float")
  .Output("selected_indices: int32")
  - .Attr("T: {half, float}")
  + .Attr("T: {half, float} = DT_FLOAT")
  .SetShapeFn([](InferenceContext* c) {
  // Get inputs and validate ranks.
  ShapeHandle boxes;
  @@ -718,7 +718,7 @@ REGISTER_OP("NonMaxSuppressionV3")
  .Input("iou_threshold: float")
  .Input("score_threshold: float")
  .Output("selected_indices: int32")
  - .Attr("T: {half, float}")
  + .Attr("T: {half, float} = DT_FLOAT")
  .SetShapeFn(NMSShapeFn);
  REGISTER_OP("NonMaxSuppressionV4")
  @@ -729,7 +729,7 @@ REGISTER_OP("NonMaxSuppressionV4")
  .Input("score_threshold: float")
  .Output("selected_indices: int32")
  .Output("valid_outputs: int32")
  - .Attr("T: {half, float}")
  + .Attr("T: {half, float} = DT_FLOAT")
  .Attr("pad_to_max_output_size: bool = false")
  .SetShapeFn([](InferenceContext* c) {
  TF_RETURN_IF_ERROR(NMSShapeFn(c));
  