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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_ssh_key():
 """Returns ssh key to connecting to cluster workers. If the env var TUNE_CLUSTER_SSH_KEY is provided, then this key will be used for syncing across different nodes. """

    path = os.environ.get("TUNE_CLUSTER_SSH_KEY",
                          os.path.expanduser("~/ray_bootstrap_key.pem"))
    if os.path.exists(path):
        return path
    return None