Test that TensorFlow is not imported on startup
_The documentation is not available anymore as the PR was closed or merged._ Should we close this PR? I'm closing this PR. Feel free to reopen it if necessary.
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