fix python version

        from collections import Mapping
    ImportError: cannot import name 'Mapping' from 'collections'

tutorials/sphinx-tutorials/pretrained_models.py

@@ -37,7 +37,7 @@
 import torch.cuda
 from tensordict.nn import TensorDictSequential
 from torch import nn
-from torchrl.envs import Compose, R3MTransform, TransformedEnv, Compose
+from torchrl.envs import Compose, R3MTransform, TransformedEnv
 from torchrl.envs.libs.gym import GymEnv
 from torchrl.modules import Actor