fix sample with replacement in case of isolated nodes

csrc/cpu/sample_cpu.cpp

@@ -63,6 +63,7 @@ sample_adj_cpu(torch::Tensor rowptr, torch::Tensor col, torch::Tensor idx,
       row_start = rowptr_data[n], row_end = rowptr_data[n + 1];
       row_count = row_end - row_start;
 
+      if (row_count > 0) {
         for (int64_t j = 0; j < num_neighbors; j++) {
           e = row_start + rand() % row_count;
           c = col_data[e];
@@ -73,6 +74,7 @@ sample_adj_cpu(torch::Tensor rowptr, torch::Tensor col, torch::Tensor idx,
           }
           cols[i].push_back(std::make_tuple(n_id_map[c], e));
         }
+      }
       out_rowptr_data[i + 1] = out_rowptr_data[i] + cols[i].size();
     }