Clean up Model Conversion Script

Currently the model conversion script will [create a bucket](https://github.com/google/JetStream/blob/main/jetstream/tools/maxtext/model_ckpt_conversion.sh#L36) `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`.