Function FillGradients

tests/cpp/tree/test_quantile_hist.cc:263–270

namespace {
void FillGradients(linalg::Matrix<GradientPair>* gpair) {
  auto h = gpair->HostView();
  for (std::size_t row = 0; row < h.Shape(0); ++row) {
    for (std::size_t target = 0; target < h.Shape(1); ++target) {
      h(row, target) = GradientPair{1.0f, 0.0f};
    }
  }
}

// Verify partitioner doesn't write past buffer end when doing
// update on small dataset after large one.