Models loaded using the from_archive method need to be saved with original config

[AkshitaB]

When allennlp train is used to fine-tune a pretrained model (model A) using from_archive(path_to_A), the finetuned model (model B) is saved with the config that contains from_archive. This means that if you try to now finetune the model B, it needs the original model A at the exact path_to_A, as well as model B. In the normal usecase, this will fail if the user does not have access to the original model A. On beaker, depending on how the code is setup, if the path to the pretrained model remains the same in experiment A -> B and experiment B -> C, it will cause a maximum recursion depth error.

Potential solution is to store the original configuration when saving a fine-tuned model (i.e., the from_archive case).

[dirkgr]

We need a Model.to_params API.