Gradient based sampling for GPU Hist (#5093)

* Implement gradient based sampling for GPU Hist tree method.
* Add samplers and handle compacted page in GPU Hist.

src/tree/gpu_hist/row_partitioner.cuh

@@ -125,7 +125,6 @@ class RowPartitioner {
       idx += segment.begin;
       RowIndexT ridx = d_ridx[idx];
       bst_node_t new_position = op(ridx);  // new node id
-      if (new_position == kIgnoredTreePosition) return;
       KERNEL_CHECK(new_position == left_nidx || new_position == right_nidx);
       AtomicIncrement(d_left_count, new_position == left_nidx);
       d_position[idx] = new_position;