Add from_pretrained to dummy timm objects
Closes https://github.com/huggingface/transformers/issues/12091

cc @NielsRogge 

@sgugger Am I missing something relative to specifying that these dummy items should be generated with the `from_pretrained` method?
Failure of the templates came from the failure of `make quality` on master, fixed in a commit, so this is good to merge!<|||||>Thanks a lot @sgugger!