TSP fix route return (rapidsai#1412)

The vertex list that was fed as input to TSP was of type `int64` which ended up being corrupted when passed down
to the cpp layer as `vtx_ptr`. I updated the wrapper to cast the vertices to `int32`. In addition, I fixed the handling of nstart in the wrapper which was assuming vertex ids were starting at 1.
Solves: rapidsai#1410

Authors:
  - Hugo Linsenmaier (@hlinsen)

Approvers:
  - Brad Rees (@BradReesWork)

URL: rapidsai#1412

python/cugraph/traversal/traveling_salesperson.py

@@ -20,7 +20,7 @@ def traveling_salesperson(pos_list,
                           restarts=100000,
                           beam_search=True,
                           k=4,
-                          nstart=1,
+                          nstart=None,
                           verbose=False,
                           ):
     """
@@ -62,7 +62,7 @@ def traveling_salesperson(pos_list,
     null_check(pos_list['x'])
     null_check(pos_list['y'])
 
-    if not pos_list[pos_list['vertex'] == nstart].index:
+    if nstart is not None and not pos_list[pos_list['vertex'] == nstart].index:
         raise ValueError("nstart should be in vertex ids")
 
     route, cost = traveling_salesperson_wrapper.traveling_salesperson(

python/cugraph/traversal/traveling_salesperson_wrapper.pyx

@@ -31,7 +31,7 @@ def traveling_salesperson(pos_list,
                           restarts=100000,
                           beam_search=True,
                           k=4,
-                          nstart=1,
+                          nstart=None,
                           verbose=False,
                           renumber=True,
 ):
@@ -43,6 +43,7 @@ def traveling_salesperson(pos_list,
     cdef uintptr_t x_pos = <uintptr_t>NULL
     cdef uintptr_t y_pos = <uintptr_t>NULL
 
+    pos_list['vertex'] = pos_list['vertex'].astype(np.int32)
     pos_list['x'] = pos_list['x'].astype(np.float32)
     pos_list['y'] = pos_list['y'].astype(np.float32)
     x_pos = pos_list['x'].__cuda_array_interface__['data'][0]
@@ -61,7 +62,10 @@ def traveling_salesperson(pos_list,
     cdef uintptr_t vtx_ptr = <uintptr_t>NULL
     vtx_ptr = pos_list['vertex'].__cuda_array_interface__['data'][0]
 
-    renumbered_nstart = pos_list[pos_list['vertex'] == nstart].index[0]
+    if nstart is None:
+      renumbered_nstart = 0
+    else:
+      renumbered_nstart = pos_list[pos_list['vertex'] == nstart].index[0]
 
     final_cost = c_traveling_salesperson(handle_[0],
             <int*> vtx_ptr,