message: Improvement of shape function in HistogramFixedWidth
  In the HistogramFixedWidth op, there are restrictions
  over the shape of range_value and nbins. The range_value
  should be a vector of 2 elements and nbins should be
  a scalar. This fix adds the restriction to shape function
  of HistogramFixedWidth.
diff: @@ -1380,6 +1380,14 @@ REGISTER_OP("HistogramFixedWidth")
  .Attr("T: {int32, int64, float32, float64}")
  .Attr("dtype: {int32, int64} = DT_INT32")
  .SetShapeFn([](InferenceContext* c) {
  + // value_range should be a vector.
  + ShapeHandle value_range_shape;
  + TF_RETURN_IF_ERROR(c->WithRank(c->input(1), 1, &value_range_shape));
  + // value_range should have two elements.
  + DimensionHandle unused;
  + TF_RETURN_IF_ERROR(c->WithValue(c->Dim(value_range_shape, 0), 2, &unused));
  +
  + // If nbins is available, set the shape from nbins.
  const Tensor* nbins_input = c->input_tensor(2);
  if (nbins_input != nullptr) {
  int64 nbins;
  