[Notebook] Fix import

Author: matteobettini

notebooks/Simulation_and_training_in_VMAS_and_BenchMARL.ipynb

@@ -1629,7 +1629,10 @@
     "TORCH = format_pytorch_version(TORCH_version)\n",
     "\n",
     "def format_cuda_version(version):\n",
-    "  return 'cu' + version.replace('.', '')\n",
+    "    if version is not None:\n",
+    "        return 'cu' + version.replace('.', '')\n",
+    "    else:\n",
+    "        return \"cpu\"\n",
     "\n",
     "CUDA_version = torch.version.cuda\n",
     "CUDA = format_cuda_version(CUDA_version)\n",