Thread safe, inplace prediction. (#5389)

Normal prediction with DMatrix is now thread safe with locks.  Added inplace prediction is lock free thread safe.

When data is on device (cupy, cudf), the returned data is also on device.

* Implementation for numpy, csr, cudf and cupy.

* Implementation for dask.

* Remove sync in simple dmatrix.

src/data/device_adapter.cuh

@@ -52,6 +52,13 @@ class CudfAdapterBatch : public detail::NoMetaInfo {
                       : std::numeric_limits<float>::quiet_NaN();
     return COOTuple(row_idx, column_idx, value);
   }
+  __device__ float GetValue(size_t ridx, bst_feature_t fidx) const {
+    auto const& column = columns_[fidx];
+    float value = column.valid.Data() == nullptr || column.valid.Check(ridx)
+                      ? column.GetElement(ridx)
+                      : std::numeric_limits<float>::quiet_NaN();
+    return value;
+  }
 
  private:
   common::Span<ArrayInterface> columns_;
@@ -129,6 +136,7 @@ class CudfAdapter : public detail::SingleBatchDataIter<CudfAdapterBatch> {
     for (auto& json_col : json_columns) {
       auto column = ArrayInterface(get<Object const>(json_col));
       columns.push_back(column);
+      CHECK_EQ(column.num_cols, 1);
       column_ptr.emplace_back(column_ptr.back() + column.num_rows);
       num_rows_ = std::max(num_rows_, size_t(column.num_rows));
       CHECK_EQ(device_idx_, dh::CudaGetPointerDevice(column.data))

src/data/simple_dmatrix.cu

@@ -122,8 +122,6 @@ SimpleDMatrix::SimpleDMatrix(AdapterT* adapter, float missing, int nthread) {
     CopyDataColumnMajor(adapter, sparse_page_.data.DeviceSpan(),
                         adapter->DeviceIdx(), missing, s_offset);
   }
-  // Sync
-  sparse_page_.data.HostVector();
 
   info.num_col_ = adapter->NumColumns();
   info.num_row_ = adapter->NumRows();