Added torch.nn import

Added `import torch.nn as nn` as students may not think to add the import half-way down the doc.

Author: jchernus

transfer-learning/Transfer_Learning_Exercise.ipynb

@@ -269,6 +269,7 @@
    },
    "outputs": [],
    "source": [
+    "import torch.nn as nn",
     "## TODO: add a last linear layer  that maps n_inputs -> 5 flower classes\n",
     "## new layers automatically have requires_grad = True\n",
     "\n",