Generalize prediction cache. (#8783)

* Extract most of the functionality into `DMatrixCache`.
* Move API entry to independent file to reduce dependency on `predictor.h` file.
* Add test.

---------

Co-authored-by: Philip Hyunsu Cho <chohyu01@cs.washington.edu>

src/common/api_entry.h

@@ -0,0 +1,35 @@
+/**
+ * Copyright 2016-2023 by XGBoost contributors
+ */
+#ifndef XGBOOST_COMMON_API_ENTRY_H_
+#define XGBOOST_COMMON_API_ENTRY_H_
+#include <string>               // std::string
+#include <vector>               // std::vector
+
+#include "xgboost/base.h"       // GradientPair,bst_ulong
+#include "xgboost/predictor.h"  // PredictionCacheEntry
+
+namespace xgboost {
+/**
+ * \brief entry to to easily hold returning information
+ */
+struct XGBAPIThreadLocalEntry {
+  /*! \brief result holder for returning string */
+  std::string ret_str;
+  /*! \brief result holder for returning raw buffer */
+  std::vector<char> ret_char_vec;
+  /*! \brief result holder for returning strings */
+  std::vector<std::string> ret_vec_str;
+  /*! \brief result holder for returning string pointers */
+  std::vector<const char *> ret_vec_charp;
+  /*! \brief returning float vector. */
+  std::vector<float> ret_vec_float;
+  /*! \brief temp variable of gradient pairs. */
+  std::vector<GradientPair> tmp_gpair;
+  /*! \brief Temp variable for returning prediction result. */
+  PredictionCacheEntry prediction_entry;
+  /*! \brief Temp variable for returning prediction shape. */
+  std::vector<bst_ulong> prediction_shape;
+};
+}  // namespace xgboost
+#endif  // XGBOOST_COMMON_API_ENTRY_H_