[Feature request] add option to set TRANSFORMERS_OFFLINE env_var to prevent unwarranted huggingface updates

[mr555ru]

This feature request is a suggestion for preventing bugs like #919.

A buggy update from Huggingface accidentally denied access to SD for the users who were under impression that they have a net-independent, locally built version of SD.

After some investigation as to why a local model can suddenly stop working without user intent or notice, I have come to a preliminary conclusion that it could be fixed by providing an option to deny internet access to transformers package and force it to run only from cache and never attempt to re-synchronize the cache.

It seems like there are roughly 2 ways to do this, outlined in this post on Huggingface:

1. Set TRANSFORMERS_OFFLINE environment variable to 1 to enable transformers offline mode.
2. Modify every call to from_pretrained() method of all transformers models and add a keyword argument local_files_only=True.

Given that it seems that calls to from_pretrained() which were affected by this bug stem from https://github.com/CompVis/stable-diffusion/ repository which is used in this fork (invoked via ldm.util.instantiate_from_config), it would suggest that an option to turn transformers_offline on in one of the scripts would be easier to implement.

ED: spelling, revised information