Offline loading
As discussed in #824 it would be cool to make the library work in offline mode.
Currently if there's not internet connection then modules (datasets or metrics) that have already been loaded in the past can't be loaded and it raises a ConnectionError.
This is because `prepare_module` fetches online for the latest version of the module.

To make it work in offline mode one suggestion was to reload the latest local version of the module.
I implemented that and I also raise a warning saying that the module that is loaded is the latest local version.
```python
logger.warning(
    f"Using the latest cached version of the module from {cached_module_path} since it "
    f"couldn't be found locally at {input_path} or remotely ({error_type_that_prevented_reaching_out_remote_stuff})."
)
```

I added tests to make sure it works as expected and I needed to do a few changes in the code to be able to test things properly. In particular I added a parameter `hf_modules_cache` to `init_dynamic_modules` for testing purposes. It makes it possible to have temporary modules caches for testing.

I also added a `offline` context utility that allows to test part of the code by making all the requests fail as if there was no internet.

Close #824, close #761.