Fix imports in conversion scripts
# What does this PR do?

During the rework of the new init for fast imports, all absolute imports were switched to relative ones indiscriminately (because they usually don't work anymore for the core of the lib). However, the conversion scripts are supposed to be executed as scripts and relative imports can't work there (that's how Python works). This PR fixes those, and it seems that it doesn't hurt the transformers-cli convert command (which import things from those modules).