Initial support for quantile loss. (#8750)

- Add support for Python.
- Add objective.

include/xgboost/linalg.h

@@ -16,6 +16,7 @@
 #include <algorithm>
 #include <cassert>
 #include <cinttypes>  // std::int32_t
+#include <cstddef>    // std::size_t
 #include <limits>
 #include <string>
 #include <tuple>
@@ -552,6 +553,11 @@ LINALG_HD auto UnravelIndex(size_t idx, common::Span<size_t const, D> shape) {
   }
 }
 
+template <size_t D>
+LINALG_HD auto UnravelIndex(size_t idx, std::size_t const (&shape)[D]) {
+  return UnravelIndex(idx, common::Span<std::size_t const, D>(shape));
+}
+
 /**
  * \brief A view over a vector, specialization of Tensor
  *