Make HostDeviceVector single gpu only (#4773)

* Make HostDeviceVector single gpu only

src/common/common.h

@@ -140,88 +140,8 @@ class Range {
   Iterator begin_;
   Iterator end_;
 };
+
+int AllVisibleGPUs();
 }  // namespace common
-
-struct AllVisibleImpl {
-  static int AllVisible();
-};
-/* \brief set of devices across which HostDeviceVector can be distributed.
- *
- * Currently implemented as a range, but can be changed later to something else,
- *   e.g. a bitset
- */
-class GPUSet {
- public:
-  using GpuIdType = int;
-  static constexpr GpuIdType kAll = -1;
-
-  explicit GPUSet(int start = 0, int ndevices = 0)
-      : devices_(start, start + ndevices) {}
-
-  static GPUSet Empty() { return GPUSet(); }
-
-  static GPUSet Range(GpuIdType start, GpuIdType n_gpus) {
-    return n_gpus <= 0 ? Empty() : GPUSet{start, n_gpus};
-  }
-  /*! \brief n_gpus and num_rows both are upper bounds. */
-  static GPUSet All(GpuIdType gpu_id, GpuIdType n_gpus,
-                    GpuIdType num_rows = std::numeric_limits<GpuIdType>::max());
-
-  static GPUSet AllVisible() {
-    GpuIdType n =  AllVisibleImpl::AllVisible();
-    return Range(0, n);
-  }
-
-  size_t Size() const {
-    GpuIdType size = *devices_.end() - *devices_.begin();
-    GpuIdType res = size < 0 ? 0 : size;
-    return static_cast<size_t>(res);
-  }
-
-  /*
-   * By default, we have two configurations of identifying device, one
-   * is the device id obtained from `cudaGetDevice'.  But we sometimes
-   * store objects that allocated one for each device in a list, which
-   * requires a zero-based index.
-   *
-   * Hence, `DeviceId' converts a zero-based index to actual device id,
-   * `Index' converts a device id to a zero-based index.
-   */
-  GpuIdType DeviceId(size_t index) const {
-    GpuIdType result = *devices_.begin() + static_cast<GpuIdType>(index);
-    CHECK(Contains(result)) << "\nDevice " << result << " is not in GPUSet."
-                            << "\nIndex: " << index
-                            << "\nGPUSet: (" << *begin() << ", " << *end() << ")"
-                            << std::endl;
-    return result;
-  }
-  size_t Index(GpuIdType device) const {
-    CHECK(Contains(device)) << "\nDevice " << device << " is not in GPUSet."
-                            << "\nGPUSet: (" << *begin() << ", " << *end() << ")"
-                            << std::endl;
-    size_t result = static_cast<size_t>(device - *devices_.begin());
-    return result;
-  }
-
-  bool IsEmpty() const { return Size() == 0; }
-
-  bool Contains(GpuIdType device) const {
-    return *devices_.begin() <= device && device < *devices_.end();
-  }
-
-  common::Range::Iterator begin() const { return devices_.begin(); }  // NOLINT
-  common::Range::Iterator end() const { return devices_.end(); }      // NOLINT
-
-  friend bool operator==(const GPUSet& lhs, const GPUSet& rhs) {
-    return lhs.devices_ == rhs.devices_;
-  }
-  friend bool operator!=(const GPUSet& lhs, const GPUSet& rhs) {
-    return !(lhs == rhs);
-  }
-
- private:
-  common::Range devices_;
-};
-
 }  // namespace xgboost
 #endif  // XGBOOST_COMMON_COMMON_H_