Create a new uniquely named run directory within the specified
runs_dir.
unique_run_dir(runs_dir = getOption("tfruns.runs_dir", "runs"))Directory containing runs. Defaults to "runs" beneath the
current working directory (or to the value of the tfruns.runs_dir R
option if specified).
The directory name will be a timestamp (in GMT time). If a duplicate name is generated then the function will wait long enough to return a unique one.