Fix Encoder-Decoder testing issue about repo. names
# What does this PR do?

Some tests for encoder-decoder models use repo. names `../gpt2`. It works before `huggingface-hub==0.10`, but failed with this version.

This PR changes `../gpt2` to `gpt2`. In any case, it is less confusing even if `../gpt2` worked before.
_The documentation is not available anymore as the PR was closed or merged._<|||||>Also flagging to @Wauplin that this is another breaking change in the last release that is not spelled out in the [release notes](https://github.com/huggingface/huggingface_hub/releases/tag/v0.10.0).<|||||>Same answer as [here](https://github.com/huggingface/transformers/pull/19252#issuecomment-1263651027).