Add PushCSC for SparsePage. (#4193)

* Add PushCSC for SparsePage.

* Move Push* definitions into cc file.
* Add std:: prefix to `size_t` make clang++ happy.
* Address monitor count == 0.

tests/cpp/data/test_data.cc

@@ -0,0 +1,55 @@
+#include <gtest/gtest.h>
+#include <vector>
+
+#include "xgboost/data.h"
+
+namespace xgboost {
+TEST(SparsePage, PushCSC) {
+  std::vector<size_t> offset {0};
+  std::vector<Entry> data;
+  SparsePage page;
+  page.offset.HostVector() = offset;
+  page.data.HostVector() = data;
+
+  offset = {0, 1, 4};
+  for (size_t i = 0; i < offset.back(); ++i) {
+    data.push_back(Entry(i, 0.1f));
+  }
+
+  SparsePage other;
+  other.offset.HostVector() = offset;
+  other.data.HostVector() = data;
+
+  page.PushCSC(other);
+
+  ASSERT_EQ(page.offset.HostVector().size(), offset.size());
+  ASSERT_EQ(page.data.HostVector().size(), data.size());
+  for (size_t i = 0; i < offset.size(); ++i) {
+    ASSERT_EQ(page.offset.HostVector()[i], offset[i]);
+  }
+  for (size_t i = 0; i < data.size(); ++i) {
+    ASSERT_EQ(page.data.HostVector()[i].index, data[i].index);
+  }
+
+  page.PushCSC(other);
+  ASSERT_EQ(page.offset.HostVector().size(), offset.size());
+  ASSERT_EQ(page.data.Size(), data.size() * 2);
+
+  for (size_t i = 0; i < offset.size(); ++i) {
+    ASSERT_EQ(page.offset.HostVector()[i], offset[i] * 2);
+  }
+
+  auto inst = page[0];
+  ASSERT_EQ(inst.size(), 2);
+  for (auto entry : inst) {
+    ASSERT_EQ(entry.index, 0);
+  }
+
+  inst = page[1];
+  ASSERT_EQ(inst.size(), 6);
+  std::vector<size_t> indices_sol {1, 2, 3};
+  for (size_t i = 0; i < inst.size(); ++i) {
+    ASSERT_EQ(inst[i].index, indices_sol[i % 3]);
+  }
+}
+}