[model_cards] BERT-of-Theseus: use the visual as thumbnail

cc @jetrunner

Co-Authored-By: Kevin Canwen Xu <canwenxu@outlook.com>

model_cards/canwenxu/BERT-of-Theseus-MNLI/README.md

@@ -1,3 +1,7 @@
+---
+thumbnail: https://github.com/JetRunner/BERT-of-Theseus/blob/master/bert-of-theseus.png?raw=true
+---
+
 # BERT-of-Theseus
 See our paper ["BERT-of-Theseus: Compressing BERT by Progressive Module Replacing"](http://arxiv.org/abs/2002.02925).