Allow compilation with nvcc 11.4 (#7131)

* Use type aliases for discard iterators

* update to include host_vector as thrust 1.12 doesn't bring it in as a side-effect

* cub::DispatchRadixSort requires signed offset types

src/tree/gpu_hist/row_partitioner.cu

@@ -61,10 +61,7 @@ struct WriteResultsFunctor {
 };
 
 // Change the value type of thrust discard iterator so we can use it with cub
-class DiscardOverload : public thrust::discard_iterator<IndexFlagTuple> {
- public:
-  using value_type = IndexFlagTuple;  // NOLINT
-};
+using DiscardOverload = thrust::discard_iterator<IndexFlagTuple>;
 
 // Implement partitioning via single scan operation using transform output to
 // write the result