[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

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