Files not accessible via IPv6
Author: leezu
Pull request: no

In certain cases (such as [1]), users only have access to the internet via IPv6. Unfortunately huggingface.co (or the domain hosting the files) does not have AAAA records and is not reachable from IPv6, causing `ValueError: Connection error, and we cannot find the requested files in the cached path. Please try again or make sure your Internet connection is on.` when triggering downloads:

```
from transformers import AutoTokenizer
AutoTokenizer.from_pretrained('facebook/mbart-large-cc25')
```

[1] https://docs.aws.amazon.com/AmazonECS/latest/developerguide/task-networking.html#task-networking-vpc-dual-stack "One of the benefits of using a VPC in dual-stack mode is that tasks that are assigned an IPv6 address are able to access the internet as long as the VPC is configured with either an internet gateway or an egress-only internet gateway. NAT gateways are not needed."

Comments:
@n1t0 might be knowledgeable about this :)

@n1t0 depending where huggingface.co is hosted, solving this issue may just boil down to turning on dual-stack support in the hosting provider's console.

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This issue hasn't been addressed 

Might be interesting for @sterchelen as well as @n1t0 

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Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.

This issue hasn't been addressed

we are looking into this

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Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.

This issue hasn't been addressed

Just commenting to keep the bot from closing the issue.

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Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.

This issue hasn't been addressed. This can only be addressed by the huggingface team.

@leezu Can you try again now?

Thank you @n1t0. I verified `AutoTokenizer.from_pretrained` and `AutoModel.from_pretrained` from an IPv6-only instance (no IPv4 route to internet) works now. Thank you for enabling dual-stack support on your end!

Yay great job on this @n1t0! You should tweet about it :)

same issure！

failed: Cannot assign requsted address.