Allow overriding the vehicle fixed cost in the initial local model fr…

…om command-line flags.

To make multiple time windows per shipment work well in the two-step routing
library, the vehicle fixed cost in the initial local model must be low, so that
the solver doesn't have a tendency to create very long routes that span across
all the time windows. Such routes do not cause any errors, but they can block
the concerned vehicle for the whole time between the two time windows.

python/cfr/two_step_routing/two_step_routing_main.py

@@ -58,6 +58,8 @@ class Flags:
     end_with_local_refinement: The value of the --end_with_local_refinement
       flag.
     local_grouping: The value of the --local_grouping flag or the default value.
+    local_model_vehicle_fixed_cost: The value of the
+      --local_model_vehicle_fixed_cost flag.
     travel_mode_in_merged_transitions: The value of the
       --travel_mode_in_merged_transitions flag.
     inject_start_times_to_refinement_first_solution: The value of the
@@ -78,6 +80,7 @@ class Flags:
   num_refinements: int
   end_with_local_refinement: bool
   local_grouping: two_step_routing.LocalModelGrouping
+  local_model_vehicle_fixed_cost: float | None
   travel_mode_in_merged_transitions: bool
   inject_start_times_to_refinement_first_solution: bool
   local_timeout: cfr_json.DurationString
@@ -119,6 +122,15 @@ def _parse_flags() -> Flags:
       help="Controls the grouping mode in the local model.",
       default=two_step_routing.LocalModelGrouping.PARKING_AND_TIME,
   )
+  parser.add_argument(
+      "--local_model_vehicle_fixed_cost",
+      default=None,
+      type=float,
+      help=(
+          "The cost of a vehicle in the initial local model. When None, the"
+          " default cost determined by the solver is used."
+      ),
+  )
   parser.add_argument(
       "--travel_mode_in_merged_transitions",
       help="Add travel mode information to transitions in the merged solution.",
@@ -206,6 +218,7 @@ def _parse_flags() -> Flags:
       google_cloud_token=flags.token,
       local_timeout=flags.local_timeout,
       local_grouping=flags.local_grouping,
+      local_model_vehicle_fixed_cost=flags.local_model_vehicle_fixed_cost,
       travel_mode_in_merged_transitions=flags.travel_mode_in_merged_transitions,
       inject_start_times_to_refinement_first_solution=flags.inject_start_times_to_refinement_first_solution,
       global_timeout=flags.global_timeout,
@@ -342,6 +355,10 @@ def _run_two_step_planner() -> None:
       raise ValueError(
           "Unexpected value of --local_grouping: {flags.local_grouping!r}"
       )
+  if flags.local_model_vehicle_fixed_cost is not None:
+    options.local_model_vehicle_fixed_cost = (
+        flags.local_model_vehicle_fixed_cost
+    )
 
   planner = two_step_routing.Planner(
       request_json, parking_locations, parking_for_shipment, options