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import launch
from launch.substitutions import Command, LaunchConfiguration
import launch_ros
import os
 
os

def generate_launch_description():
 pkg_share = launch_ros.substitutions.FindPackageShare(package='test_robot').find('test_robot')
 default_model_path = os.path.join(pkg_share, 'urdf/test_robot.xacro')
 default_with_gripper = 'False'
 
'False'

robot_state_publisher_node = launch_ros.actions.Node(
    package='robot_state_publisher',
    executable='robot_state_publisher',
    parameters=[{'robot_description': Command(['xacro ', LaunchConfiguration('model'), ' with_gripper:=', LaunchConfiguration('spawn_with_gripper')])}]
)
return launch.LaunchDescription([
    launch.actions.DeclareLaunchArgument(name='model', default_value=default_model_path,
                                        description='Absolute path to robot urdf file'),
    launch.actions.DeclareLaunchArgument(name='spawn_with_gripper', default_value=default_with_gripper,
                                description='Spawn xacro with gripper?'),
    robot_state_publisher_node,
])
 
edit: I did not understand the full question, the below is not a satisfactory answer
 
I think the robot_param:= is where you are having the issue. 
 
You should be able to do the following directly:
 
    robot_state_publisher_node = launch_ros.actions.Node(
    package='robot_state_publisher',
    executable='robot_state_publisher',
    parameters=[{'robot_description': Command(['xacro ', <path_to_your_xacro_here>])}]
)
 
With the following imports
 
import launch
from launch.substitutions import Command, LaunchConfiguration
import launch_ros
import os