Replaced std::vector with HostDeviceVector in MetaInfo and SparsePage. (#3446)

* Replaced std::vector with HostDeviceVector in MetaInfo and SparsePage.

- added distributions to HostDeviceVector
- using HostDeviceVector for labels, weights and base margings in MetaInfo
- using HostDeviceVector for offset and data in SparsePage
- other necessary refactoring

* Added const version of HostDeviceVector API calls.

- const versions added to calls that can trigger data transfers, e.g. DevicePointer()
- updated the code that uses HostDeviceVector
- objective functions now accept const HostDeviceVector<bst_float>& for predictions

* Updated src/linear/updater_gpu_coordinate.cu.

* Added read-only state for HostDeviceVector sync.

- this means no copies are performed if both host and devices access
  the HostDeviceVector read-only

* Fixed linter and test errors.

- updated the lz4 plugin
- added ConstDeviceSpan to HostDeviceVector
- using device % dh::NVisibleDevices() for the physical device number,
  e.g. in calls to cudaSetDevice()

* Fixed explicit template instantiation errors for HostDeviceVector.

- replaced HostDeviceVector<unsigned int> with HostDeviceVector<int>

* Fixed HostDeviceVector tests that require multiple GPUs.

- added a mock set device handler; when set, it is called instead of cudaSetDevice()

tests/cpp/data/test_simple_dmatrix.cc

@@ -13,7 +13,7 @@ TEST(SimpleDMatrix, MetaInfo) {
   EXPECT_EQ(dmat->Info().num_row_, 2);
   EXPECT_EQ(dmat->Info().num_col_, 5);
   EXPECT_EQ(dmat->Info().num_nonzero_, 6);
-  EXPECT_EQ(dmat->Info().labels_.size(), dmat->Info().num_row_);
+  EXPECT_EQ(dmat->Info().labels_.Size(), dmat->Info().num_row_);
 
   delete dmat;
 }