use latest saved weights by default in tutorial

Use the latest saved weights automatically for coc0_eval.py but also allow users to specify their own choice of weight file.

Author: shreyas-bk

tutorial/train_shape.ipynb

@@ -469,7 +469,7 @@
         "%cd /content/Yet-Another-EfficientDet-Pytorch\n",
         "\n",
         "#uncomment the next line to specify a weight file\n",
-        "#weight_file[0] = 'logs/shape/efficientdet-d0_49_1400.pth'\n",
+        "#weight_file[0] = 'efficientdet-d0_49_1400.pth'\n",
         "\n",
         "! python coco_eval.py -c 0 -p shape -w \"logs/shape/{weight_file[0]}\""
       ],