code: def valid_processor_options(processors=None):
      
      if processors is None:
          processors = [
              dynamic_import(p) for p in
              tuple(settings.THUMBNAIL_PROCESSORS) +
              tuple(settings.THUMBNAIL_SOURCE_GENERATORS)]
      valid_options = set(['size', 'quality', 'subsampling'])
      for processor in processors:
          args = inspect.getfullargspec(processor)[0] if six.PY3 else inspect.getargspec(processor)[0]
          # Add all arguments apart from the first (the source image).
          valid_options.update(args[1:])
      return list(valid_options)
docs: Return a list of unique valid options for a list of image processors
      (and/or source generators)