Test that TensorFlow is not imported on startup
TF takes some time to be imported, and also uses some GPU memory.

I just added a test to make sure that in the future it's never imported by default when
```python
import datasets
```
is called.

Right now this fails because `huggingface_hub` does import tensorflow (though this is fixed now on their `main` branch)

I'll mark this PR as ready for review once `huggingface_hub` has a new release
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_4482). All of your documentation changes will be reflected on that endpoint.