Rework the MAP metric. (#8931)

- The new implementation is more strict as only binary labels are accepted. The previous implementation converts values greater than 1 to 1.
- Deterministic GPU. (no atomic add).
- Fix top-k handling.
- Precise definition of MAP. (There are other variants on how to handle top-k).
- Refactor GPU ranking tests.

tests/cpp/common/test_ranking_utils.cc

@@ -177,4 +177,36 @@ TEST(NDCGCache, InitFromCPU) {
   Context ctx;
   TestNDCGCache(&ctx);
 }
+
+void TestMAPCache(Context const* ctx) {
+  auto p_fmat = EmptyDMatrix();
+  MetaInfo& info = p_fmat->Info();
+  LambdaRankParam param;
+  param.UpdateAllowUnknown(Args{});
+
+  std::vector<float> h_data(32);
+
+  common::Iota(ctx, h_data.begin(), h_data.end(), 0.0f);
+  info.labels.Reshape(h_data.size());
+  info.num_row_ = h_data.size();
+  info.labels.Data()->HostVector() = std::move(h_data);
+
+  auto fail = [&]() { std::make_shared<MAPCache>(ctx, info, param); };
+  // binary label
+  ASSERT_THROW(fail(), dmlc::Error);
+
+  h_data = std::vector<float>(32, 0.0f);
+  h_data[1] = 1.0f;
+  info.labels.Data()->HostVector() = h_data;
+  auto p_cache = std::make_shared<MAPCache>(ctx, info, param);
+
+  ASSERT_EQ(p_cache->Acc(ctx).size(), info.num_row_);
+  ASSERT_EQ(p_cache->NumRelevant(ctx).size(), info.num_row_);
+}
+
+TEST(MAPCache, InitFromCPU) {
+  Context ctx;
+  ctx.Init(Args{});
+  TestMAPCache(&ctx);
+}
 }  // namespace xgboost::ltr

tests/cpp/common/test_ranking_utils.cu

@@ -95,4 +95,10 @@ TEST(NDCGCache, InitFromGPU) {
   ctx.UpdateAllowUnknown(Args{{"gpu_id", "0"}});
   TestNDCGCache(&ctx);
 }
+
+TEST(MAPCache, InitFromGPU) {
+  Context ctx;
+  ctx.UpdateAllowUnknown(Args{{"gpu_id", "0"}});
+  TestMAPCache(&ctx);
+}
 }  // namespace xgboost::ltr

tests/cpp/common/test_ranking_utils.h

@@ -6,4 +6,6 @@
 
 namespace xgboost::ltr {
 void TestNDCGCache(Context const* ctx);
+
+void TestMAPCache(Context const* ctx);
 }  // namespace xgboost::ltr