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>

tests/cpp/test_cache.cc

@@ -0,0 +1,55 @@
+/**
+ * Copyright 2023 by XGBoost contributors
+ */
+#include <gtest/gtest.h>
+#include <xgboost/cache.h>
+#include <xgboost/data.h>  // DMatrix
+
+#include <cstddef>         // std::size_t
+
+#include "helpers.h"       // RandomDataGenerator
+
+namespace xgboost {
+namespace {
+struct CacheForTest {
+  std::size_t i;
+
+  explicit CacheForTest(std::size_t k) : i{k} {}
+};
+}  // namespace
+
+TEST(DMatrixCache, Basic) {
+  std::size_t constexpr kRows = 2, kCols = 1, kCacheSize = 4;
+  DMatrixCache<CacheForTest> cache(kCacheSize);
+
+  auto add_cache = [&]() {
+    // Create a lambda function here, so that p_fmat gets deleted upon the
+    // end of the lambda. This is to test how the cache handle expired
+    // cache entries.
+    auto p_fmat = RandomDataGenerator(kRows, kCols, 0).GenerateDMatrix();
+    cache.CacheItem(p_fmat, 3);
+    DMatrix* m = p_fmat.get();
+    return m;
+  };
+  auto m = add_cache();
+  ASSERT_EQ(cache.Container().size(), 0);
+  ASSERT_THROW(cache.Entry(m), dmlc::Error);
+
+  auto p_fmat = RandomDataGenerator(kRows, kCols, 0).GenerateDMatrix();
+
+  auto item = cache.CacheItem(p_fmat, 1);
+  ASSERT_EQ(cache.Entry(p_fmat.get())->i, 1);
+
+  std::vector<std::shared_ptr<DMatrix>> items;
+  for (std::size_t i = 0; i < kCacheSize * 2; ++i) {
+    items.emplace_back(RandomDataGenerator(kRows, kCols, 0).GenerateDMatrix());
+    cache.CacheItem(items.back(), i);
+    ASSERT_EQ(cache.Entry(items.back().get())->i, i);
+    ASSERT_LE(cache.Container().size(), kCacheSize);
+    if (i > kCacheSize) {
+      auto k = i - kCacheSize - 1;
+      ASSERT_THROW(cache.Entry(items[k].get()), dmlc::Error);
+    }
+  }
+}
+}  // namespace xgboost