Consolidate directories for writing out files

[neubig]

Currently prompt2model writes out many files, which can be basically split into two categories:

Cache

These are all of the files that are used by the various models that generate the result. These should include:

• dataset_info.json
• model_info.json
• The retrieval indices generated from these two files

These should be written to a cache directory, specified by the following function:

def get_prompt2model_cache_dir() -> str:
    """Get the cache directory for prompt2model."""
    default_cache = os.path.join(os.environ["HOME"], ".cache", "prompt2model")
    return os.environ.get("PROMPT2MODEL_CACHE", default_cache)

Experiment outputs

These are outputs of a particular experiment. It would be good if the default directories for these outputs could be consolidated to all be beneath a single directory, such as output or something. Currently the results are output to several different directories, which is confusing.