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/metric/test_rank_metric.cc

@@ -141,7 +141,7 @@ TEST(Metric, DeclareUnifiedTest(MAP)) {
   // Rank metric with group info
   EXPECT_NEAR(GetMetricEval(metric,
                             {0.1f, 0.9f, 0.2f, 0.8f, 0.4f, 1.7f},
-                            {2, 7, 1, 0, 5, 0},  // Labels
+                            {1, 1, 1, 0, 1, 0},  // Labels
                             {},  // Weights
                             {0, 2, 5, 6}),  // Group info
               0.8611f, 0.001f);