gazebo_ros_laser and tf questions

edit

In my case I just make the "use_sim_time" value "true" then the base_link-> laser_link, base_link->imu_link and base_footprint->base_link start publishing the transform. You can see the scan topic from odom frame. This is how my launch file look like.

<launch>
  <!-- overwriting these args -->
  <arg name="debug" default="false" />
  <arg name="gui" default="true" />
  <arg name="pause" default="false" />
  <arg name="world" default="$(find my_simulations)/world/empty_world.world" />

  <!--Gazebo empty world launch file-->
  <include file="$(find gazebo_ros)/launch/empty_world.launch">
        <arg name="world_name" value="$(arg world)" />
        <arg name="debug" value="false" />
        <arg name="gui" value="true" />
        <arg name="paused" value="false"/>
        <arg name="use_sim_time" value="true"/>
        <arg name="headless" value="false"/>
        <arg name="verbose" value="true"/>
  </include>

  <!--Robot Description from URDF-->
  <param name="robot_description" command="$(find xacro)/xacro --inorder '$(find robot_description)/urdf/mybot.xacro'" />
  <param name="use_gui" value="false"/>

  <!--Nodes-->
  <node name="joint_state_publisher" pkg="joint_state_publisher" type="joint_state_publisher"/>
  <node name="robot_state_publisher" pkg="robot_state_publisher" type="state_publisher"/>
  <node name="spawn_model" pkg="gazebo_ros" type="spawn_model" args="-urdf -param robot_description -model mybot -verbose" output="screen"/>
  <node name="rviz" pkg="rviz" type="rviz" args="-d $(find cobot_description)/rviz/mybot.rviz" required="true"/>

</launch>