Colab session crashes on transformers
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## Details
Hi everybody, 
Something really strange happens in Colab after the recent updates of the DataCollate class.
I don't know if the two things are correlated, however, after I install the following packages

!git clone https://github.com/huggingface/transformers.git
!pip install ./transformers
!pip install -U nlp

and then I try to load them

import nlp
from transformers import T5Tokenizer

The Colab instance crashes.
Please find below the log.

**WARNING:root:kernel 485c962d-efa0-4103-9c68-ed22abd8839f restarted**



The same thing for me. Colab (using TPU) crashes when importing transformers package.
![image](https://user-images.githubusercontent.com/35801846/84882301-ee662800-b086-11ea-9a1c-ff07853af864.png)

![image](https://user-images.githubusercontent.com/35801846/84882177-c5459780-b086-11ea-8c47-4b06eb3e8249.png)
<|||||>Same issue.<|||||>I'm trying to reproduce, but can't manage to make colab crash. @khalilRhouma, @amitness, did you have similar code to @antoniomastro1996? Would it be possible for you to show me the code you used?<|||||>@LysandreJik yes, of course you can follow this:
https://colab.research.google.com/drive/1jwXgtOXE8v8_qkiOCbjFQRFC5semK8T7?usp=sharing<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
