Extract optional weight. (#8747)

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

src/common/common.h

@@ -1,5 +1,5 @@
-/*!
- * Copyright 2015-2022 by XGBoost Contributors
+/**
+ * Copyright 2015-2023 by XGBoost Contributors
  * \file common.h
  * \brief Common utilities
  */
@@ -199,17 +199,6 @@ std::vector<Idx> ArgSort(Container const &array, Comp comp = std::less<V>{}) {
   return result;
 }
 
-struct OptionalWeights {
-  Span<float const> weights;
-  float dft{1.0f};  // fixme: make this compile time constant
-
-  explicit OptionalWeights(Span<float const> w) : weights{w} {}
-  explicit OptionalWeights(float w) : dft{w} {}
-
-  XGBOOST_DEVICE float operator[](size_t i) const { return weights.empty() ? dft : weights[i]; }
-  auto Empty() const { return weights.empty(); }
-};
-
 /**
  * Last index of a group in a CSR style of index pointer.
  */