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

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

src/common/common.h

@@ -6,6 +6,8 @@
 #ifndef XGBOOST_COMMON_COMMON_H_
 #define XGBOOST_COMMON_COMMON_H_
 
+#include <xgboost/base.h>
+
 #include <vector>
 #include <string>
 #include <sstream>
@@ -35,6 +37,71 @@ inline std::string ToString(const T& data) {
   return os.str();
 }
 
+/*
+ * Range iterator
+ */
+class Range {
+ public:
+  class Iterator {
+    friend class Range;
+
+   public:
+    using DifferenceType = int64_t;
+
+    XGBOOST_DEVICE int64_t operator*() const { return i_; }
+    XGBOOST_DEVICE const Iterator &operator++() {
+      i_ += step_;
+      return *this;
+    }
+    XGBOOST_DEVICE Iterator operator++(int) {
+      Iterator res {*this};
+      i_ += step_;
+      return res;
+    }
+
+    XGBOOST_DEVICE bool operator==(const Iterator &other) const {
+      return i_ >= other.i_;
+    }
+    XGBOOST_DEVICE bool operator!=(const Iterator &other) const {
+      return i_ < other.i_;
+    }
+
+    XGBOOST_DEVICE void Step(DifferenceType s) { step_ = s; }
+
+   protected:
+    XGBOOST_DEVICE explicit Iterator(int64_t start) : i_(start) {}
+    XGBOOST_DEVICE explicit Iterator(int64_t start, int step) :
+        i_{start}, step_{step} {}
+
+   public:
+    int64_t i_;
+    DifferenceType step_ = 1;
+  };
+
+  XGBOOST_DEVICE Iterator begin() const { return begin_; }  // NOLINT
+  XGBOOST_DEVICE Iterator end() const { return end_; }      // NOLINT
+
+  XGBOOST_DEVICE Range(int64_t begin, int64_t end)
+      : begin_(begin), end_(end) {}
+  XGBOOST_DEVICE Range(int64_t begin, int64_t end, Iterator::DifferenceType step)
+      : begin_(begin, step), end_(end) {}
+
+  XGBOOST_DEVICE bool operator==(const Range& other) const {
+    return *begin_ == *other.begin_ && *end_ == *other.end_;
+  }
+  XGBOOST_DEVICE bool operator!=(const Range& other) const {
+    return !(*this == other);
+  }
+
+  XGBOOST_DEVICE void Step(Iterator::DifferenceType s) { begin_.Step(s); }
+
+  XGBOOST_DEVICE Iterator::DifferenceType GetStep() const { return begin_.step_; }
+
+ private:
+  Iterator begin_;
+  Iterator end_;
+};
+
 }  // namespace common
 }  // namespace xgboost
 #endif  // XGBOOST_COMMON_COMMON_H_