Apply suggestions from code review

Author: alejoe91

src/spikeinterface/sorters/external/kilosort4.py

@@ -438,9 +438,9 @@ def _setup_json_probe_map(cls, recording, sorter_output_folder):
         chanMap = np.arange(n_chan)
         xc = positions[:, 0]
         yc = positions[:, 1]
-        unique_groups = np.unique(groups)
+        unique_groups = set(groups)
         group_map = {group: idx for idx, group in enumerate(unique_groups)}
-        kcoords = np.array([group_map[group] for group in groups], dtype=int)
+        kcoords = np.array(group_map.values(), dtype=int)
 
         probe = {
             "chanMap": chanMap,