TensorFlow tests: having from_pt set to True requires torch to be installed.
Some tests were executed without having torch installed, while they require torch. Namely, all the tests that have a `from_pt=True` requirement require torch to be installed.

This is a draft PR as several of the requirements to merge this PR are not met:
- The Marian models do not have their tensorflow variant available on the hub
- Neither do the RAG models

The easy option is to only set `@requires_torch`, but since we have no slow test suite that runs both PT + TF that's not a good workaround.

How do you want to proceed @patrickvonplaten ?

@LysandreJik - I'll upload the respective weights today and then check that all these slow tests here work without `from_pt`<|||||>Uploaded all the TF weights and checked that:

`RUN_SLOW=1 pytest tests/test_modeling_tf_rag.py`
`RUN_SLOW=1 pytest tests/test_modeling_tf_blenderbot.py` 

pass.

For some reason `RUN_SLOW=1 pytest tests/test_modeling_tf_marian.py` now throws an error. I've opened a new issue for this here: https://github.com/huggingface/transformers/issues/10680