[dask] Test for data initializaton. (#6226)

src/common/quantile.cu

@@ -369,6 +369,7 @@ size_t SketchContainer::ScanInput(Span<SketchEntry> entries, Span<OffsetT> d_col
    * from user input data.  Second is duplicated sketching entries, which is generated by
    * prunning or merging. We preserve the first type and remove the second type.
    */
+  timer_.Start(__func__);
   dh::safe_cuda(cudaSetDevice(device_));
   CHECK_EQ(d_columns_ptr_in.size(), num_columns_ + 1);
   dh::XGBCachingDeviceAllocator<char> alloc;