use latest saved weight in tutorial

Use latest saved weight by default in tutorial but also allow the user to specify a weight file.

Author: shreyas-bk

tutorial/train_shape.ipynb

@@ -466,7 +466,7 @@
         "#get latest weight file\n",
         "%cd logs/shape\n",
         "weight_file = !ls -Art | tail -n 1\n",
-        "%cd -\n",
+        "%cd ../..\n",
         "\n",
         "#uncomment the next line to specify a weight file\n",
         "#weight_file[0] = 'efficientdet-d0_49_1400.pth'\n",