Merge generic device helper functions into gpu set. (#3626)

* Remove the use of old NDevices* functions.
* Use GPUSet in timer.h.

tests/cpp/common/test_gpuset.cc

@@ -0,0 +1,37 @@
+#include "../../../src/common/gpu_set.h"
+#include <gtest/gtest.h>
+
+namespace xgboost {
+
+TEST(GPUSet, Basic) {
+  GPUSet devices = GPUSet::Empty();
+  ASSERT_TRUE(devices.IsEmpty());
+
+  devices = GPUSet{0, 1};
+  ASSERT_TRUE(devices != GPUSet::Empty());
+  EXPECT_EQ(devices.Size(), 1);
+
+  EXPECT_ANY_THROW(devices.Index(1));
+  EXPECT_ANY_THROW(devices.Index(-1));
+
+  devices = GPUSet::Range(1, 0);
+  EXPECT_EQ(devices, GPUSet::Empty());
+  EXPECT_EQ(devices.Size(), 0);
+  EXPECT_TRUE(devices.IsEmpty());
+
+  EXPECT_FALSE(devices.Contains(1));
+
+  devices = GPUSet::Range(2, -1);
+  EXPECT_EQ(devices, GPUSet::Empty());
+  EXPECT_EQ(devices.Size(), 0);
+  EXPECT_TRUE(devices.IsEmpty());
+
+  devices = GPUSet::Range(2, 8);
+  EXPECT_EQ(devices.Size(), 8);
+  devices = devices.Unnormalised();
+
+  EXPECT_EQ(*devices.begin(), 0);
+  EXPECT_EQ(*devices.end(), devices.Size());
+}
+
+}  // namespace xgboost