code: def fetch_artifact(self, trial_id, prefix):
          
          # TODO: general windows concern: local prefix will be in
          # backslashes but remote dirs will be expecting /
          # TODO: having s3 logic split between project and sync.py
          # worries me
          local = os.path.join(self.log_dir, trial_id, prefix)
          if self.upload_dir:
              remote = '/'.join([self.upload_dir, trial_id, prefix])
              _remote_to_local_sync(remote, local)
          return local
docs: Verifies that all children of the artifact prefix path are
          available locally. Fetches them if not.
  
          Returns the local path to the given trial's artifacts at the
          specified prefix, which is always just
  
          {log_dir}/{trial_id}/{prefix}