Handle serialization/loading of ConfigHolder with missing values

[rusheb]

If ConfigHolder has missing values it will fail to serialize, as demonstrated in test_serialize_and_load_missing_values.

This might be an issue if we don't have all the configs, e.g. at the dataset creation stage.

However maybe at that point we wouldn't serialize the entire config. I'm not sure what the best behaviour should be

[valedan]

This isn't causing any problems currently because during dataset creation we don't use ConfigHolder, just the dataset config.

I'm not really sure what our long-term plans are with configs and ConfigHolder specifically, so I can't say if this is something we should work on or not. @mivanit do you have any input here?

[mivanit]

If we aren't currently using ConfigHolder with missing values anywhere, I think its best to have it throw an exception if we try to serialize one with missing values.

[valedan]

Sounds reasonable. In that case the only TODO for this ticket would be to delete the test that @rusheb linked.

[mivanit]

I haven't had any issues with this for a while, despite several changes to the ConfigHolder (i.e. maze_tokenizer attr in #191 ). Key point is to make sure you allow for missing values in the loading_fn