T ** 2 in distillation process
Hello, 

I am confused about one part in the https://github.com/huggingface/transformers/blob/master/examples/research_projects/distillation/distiller.py script by VictorSanh et. al. 

![image](https://user-images.githubusercontent.com/33768245/122886299-2100cb80-d340-11eb-8118-0c0888c05fb1.png)

Why does the script weight the KL-Divergence between the student and the teacher distribution with an additional `T ** 2` namely the ```* (self.temperature) ** 2``` part in line 417.
The Hinton-paper says something about weighting with the squared temperature value but in this blog post: https://medium.com/huggingface/distilbert-8cf3380435b5 by VictorSanh (the same author) the KL value (can be found in about the middle of the page) does not seem to be weighted.

What am I missing here? Thank you.

@VictorSanh 
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