Allow freezing from config

[TimotheeMickus]

Given the scope of experiments, it should be enough to have a global flag in the config, and then after the encoder is created here: https://github.com/Helsinki-NLP/mammoth/blob/1e6b03a6ce3853131d093d73bf91fdad1ed2a48d/mammoth/model_builder.py#L361

iterate over its parameters and requires_grad=False them