Config supports nested string formatting

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Format strings that are nested in lists

# load SOLVER.MAX_ITER as a tag
cfg.DESCRIPTION.TAGS = ["tag1", f"{cfg.SOLVER.MAX_ITER}"]

Changes:

• set_recursive can be called with list indexing
  • e.g. cfg.set_recursive("cfg.DESCRIPTION.TAGS[0]", "new_tag")

What doesnt work:

• We can't updated nested values in dictionaries
  • e.g. set_recursive("cfg.DESCRIPTION.TAGS[0][0]", "new_value")