[BUG] Replay buffer crashing in example dreamer.py

Describe the bug

https://github.com/pytorch/rl/blob/main/examples/dreamer/dreamer.py
In the dreamer example the line 234 replay_buffer.extend(tensordict.cpu()) cause this bug

Creating a MemmapStorage...
The storage is being created: 
/Tmp/anaconda3/envs/torchrl/lib/python3.9/multiprocessing/resource_tracker.py:216: UserWarning: resource_tracker: There appear to be 1 leaked semaphore objects to clean up at shutdown
  warnings.warn('resource_tracker: There appear to be %d '
Killed

To Reproduce

Run dreamer.py with python3.9 or 3.10

Expected behavior

The script to be running normally

Reason and Possible fixes

From what I've read, this bug is occurring in python3.9/3.10 with multiprocessing. I haven't found a way to jump around it any suggestion ? Also, I would like to request to make the examples in the tutorial more suited for a single simple GPU run since I think most of the people that will be learning the library using this example.
Thank you very much.

Checklist

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