Fix IsDense. (#5702)

tests/cpp/data/test_device_dmatrix.cu

@@ -129,3 +129,22 @@ TEST(DeviceDMatrix, Equivalent) {
     }
   }
 }
+
+TEST(DeviceDMatrix, IsDense) {
+  int num_bins = 16;
+  auto test = [num_bins] (float sparsity) {
+    HostDeviceVector<float> data;
+    std::string interface_str = RandomDataGenerator{10, 10, sparsity}
+      .Device(0).GenerateArrayInterface(&data);
+    data::CupyAdapter x{interface_str};
+    std::unique_ptr<data::DeviceDMatrix> device_dmat{ new data::DeviceDMatrix(
+        &x, std::numeric_limits<float>::quiet_NaN(), 1, num_bins) };
+    if (sparsity == 0.0) {
+      ASSERT_TRUE(device_dmat->IsDense()) << sparsity;
+    } else {
+      ASSERT_FALSE(device_dmat->IsDense());
+    }
+  };
+  test(0.0);
+  test(0.1);
+}