Added necessary import multiprocessing (#2826)

* Added necessary import multiprocessing

---------

Co-authored-by: Svetlana Karslioglu <[email protected]>

Author: mudPi314

intermediate_source/reinforcement_ppo.py

@@ -104,12 +104,12 @@
 # description and more about the algorithm itself.
 #
 
-# sphinx_gallery_start_ignore
 import warnings
-
 warnings.filterwarnings("ignore")
 from torch import multiprocessing
 
+# sphinx_gallery_start_ignore
+
 # TorchRL prefers spawn method, that restricts creation of  ``~torchrl.envs.ParallelEnv`` inside
 # `__main__` method call, but for the easy of reading the code switch to fork
 # which is also a default spawn method in Google's Colaboratory