add contributor and change difficulty

Author: komaksym

questions/195_gradient-checkpointing/meta.json

@@ -1,10 +1,10 @@
-c
+{
   "id": "195",
   "title": "Gradient Checkpointing",
-  "difficulty": "medium",
+  "difficulty": "easy",
   "category": "Machine Learning",
   "video": "",
   "likes": "0",
   "dislikes": "0",
-  "contributor": []
+  "contributor": ["https://github.com/komaksym"]
 }