fixed n_file bug in plan_basis

csst_dag/dag/_dispatcher.py

@@ -177,6 +177,7 @@ class Dispatcher:
                     else:
                         this_n_file = len(_["params"]["exposure_start"])
                 else:
+                    # count effective detectors of this instrument
                     this_n_file = len(csst[this_instrument].effective_detector_names)
                 _["n_file"] = this_n_file
         except KeyError: