BFS fix for exec_policy (#4417)

Shouldn't be constructing rmm::exec_policy from another rmm::exec_policy.  Recent raft change causes this to not compile, but we shouldn't have been doing this in the first place.

Authors:
  - Chuck Hastings (https://github.com/ChuckHastings)

Approvers:
  - Seunghwa Kang (https://github.com/seunghwak)
  - Naim (https://github.com/naimnv)

URL: #4417

cpp/src/traversal/bfs_impl.cuh

@@ -31,8 +31,6 @@
 
 #include <raft/core/handle.hpp>
 
-#include <rmm/exec_policy.hpp>
-
 #include <thrust/copy.h>
 #include <thrust/count.h>
 #include <thrust/fill.h>
@@ -149,19 +147,19 @@ void bfs(raft::handle_t const& handle,
   auto constexpr invalid_distance = std::numeric_limits<vertex_t>::max();
   auto constexpr invalid_vertex   = invalid_vertex_id<vertex_t>::value;
 
-  thrust::fill(rmm::exec_policy(handle.get_thrust_policy()),
+  thrust::fill(handle.get_thrust_policy(),
                distances,
                distances + push_graph_view.local_vertex_partition_range_size(),
                invalid_distance);
-  thrust::fill(rmm::exec_policy(handle.get_thrust_policy()),
+  thrust::fill(handle.get_thrust_policy(),
                predecessor_first,
                predecessor_first + push_graph_view.local_vertex_partition_range_size(),
                invalid_vertex);
   auto vertex_partition = vertex_partition_device_view_t<vertex_t, GraphViewType::is_multi_gpu>(
     push_graph_view.local_vertex_partition_view());
   if (n_sources) {
     thrust::for_each(
-      rmm::exec_policy(handle.get_thrust_policy()),
+      handle.get_thrust_policy(),
       sources,
       sources + n_sources,
       [vertex_partition, distances, predecessor_first] __device__(auto v) {