Removing datamodel_code_generator from main installation step (#375)

* Using a fixed version of datamodel_code_generator and install as its own step

Signed-off-by: Aaron Chong <aaronchongth@gmail.com>

* Remove datamodel_code_generator from main instructions

Signed-off-by: Aaron Chong <aaronchongth@gmail.com>

---------

Signed-off-by: Aaron Chong <aaronchongth@gmail.com>

Author: aaronchongth

README.md

@@ -46,7 +46,8 @@ sudo apt update && sudo apt install \
 sudo sh -c 'echo "deb http://packages.osrfoundation.org/gazebo/ubuntu-stable `lsb_release -cs` main" > /etc/apt/sources.list.d/gazebo-stable.list'
 wget https://packages.osrfoundation.org/gazebo.key -O - | sudo apt-key add -
 
-python3 -m pip install flask-socketio fastapi uvicorn datamodel_code_generator
+# These pip packages are only used by rmf_demos which are not released as binaries
+python3 -m pip install flask-socketio fastapi uvicorn
 ```
 
 `rosdep` helps install dependencies for ROS packages across various distros and will be installed along with `ros-dev-tools`.