what is the use of " .format(**config)) "

edit

I'm assuming that you mean

rospy.loginfo("Config set to {int_param}, {double_param}, {str_param}, {bool_param}, {size}".format(**config))

This is just plain Python. From the Python docs

str.format(*args, **kwargs)

Perform a string formatting operation. The string on which this method is called can contain literal text or replacement fields delimited by braces {}. Each replacement field contains either the numeric index of a positional argument, or the name of a keyword argument. Returns a copy of the string where each replacement field is replaced with the string value of the corresponding argument.

As for the **config, checkout this answer on StackOverlfow:

This is a shortcut that allows you to pass multiple arguments to a function directly using either a list/tuple or a dictionary.

As for using it with numpy, I'm not too familiar with numpy but I don't believe that you do. But, you should ask a new question about that specifically.