Fix add-new-model-like when old model checkpoint is not found
# What does this PR do?

This PR fixes the add-new-model-like command when the old model checkpoint can't be read in its modeling file (because there is no `_CHECKPOINT_FOR_DOC` basically). In this case, the user will be asked to provide it to avoid random failures like when trying to build a new model from rag.
_The documentation is not available anymore as the PR was closed or merged._