Fix several GPU bugs (#2916)

* Fix #2905

* Fix gpu_exact test failures

* Fix bug in GPU prediction where multiple calls to batch prediction can produce incorrect results

* Fix GPU documentation formatting

src/tree/updater_gpu_hist.cu

@@ -287,6 +287,10 @@ struct DeviceShard {
     size_t compressed_size_bytes =
         common::CompressedBufferWriter::CalculateBufferSize(
             ellpack_matrix.size(), num_symbols);
+
+    CHECK(!(param.max_leaves == 0 && param.max_depth == 0))
+        << "Max leaves and max depth cannot both be unconstrained for "
+           "gpu_hist.";
     int max_nodes =
         param.max_leaves > 0 ? param.max_leaves * 2 : n_nodes(param.max_depth);
     ba.allocate(device_idx, param.silent, &gidx_buffer, compressed_size_bytes,