Extract optional weight. (#8747)

- Extract optional weight from coommon.h to reduce dependency on this header.
- Add test.

tests/cpp/common/test_optional_weight.cc

@@ -0,0 +1,24 @@
+/**
+ * Copyright 2023 by XGBoost Contributors
+ */
+#include <gtest/gtest.h>
+#include <xgboost/context.h>             // Context
+#include <xgboost/host_device_vector.h>  // HostDeviceVector
+
+#include "../../../src/common/optional_weight.h"
+namespace xgboost {
+namespace common {
+TEST(OptionalWeight, Basic) {
+  HostDeviceVector<float> weight{{2.0f, 3.0f, 4.0f}};
+  Context ctx;
+  auto opt_w = MakeOptionalWeights(&ctx, weight);
+  ASSERT_EQ(opt_w[0], 2.0f);
+  ASSERT_FALSE(opt_w.Empty());
+
+  weight.HostVector().clear();
+  opt_w = MakeOptionalWeights(&ctx, weight);
+  ASSERT_EQ(opt_w[0], 1.0f);
+  ASSERT_TRUE(opt_w.Empty());
+}
+}  // namespace common
+}  // namespace xgboost