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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_group(self):
 """Get the group of the Dataset. Returns ------- group : numpy array or None Group size of each group. """

        if self.group is None:
            self.group = self.get_field('group')
            if self.group is not None:
                # group data from LightGBM is boundaries data, need to convert to group size
                self.group = np.diff(self.group)
        return self.group