Clean up Model Conversion Script

[yeandy]

Currently the model conversion script will create a bucket export MODEL_BUCKET=gs://${USER}-maxtext. However, it may be the case that the gs://${USER}-maxtext path already exists, which I imagine would break the script.

Solution: Be able to read in a few more arguments MODEL_BUCKET and BASE_OUTPUT_DIRECTORY. We should also delete references to DATASET_PATH.