Clarify that outputs are written out to current directory if not specified

[desilinguist]

Right now, if we do not specify predictions or log or models in the configuration file, they are still written out but in the current directory where the experiments are run. This is not clear from the documentation. We need to be consistent and either:

1. Update the documentation to clearly state what is written out in the current directory.
2. Modify the code to note write out artifacts at all if the user explicitly does not specify a particular output directory.