<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _drop_diagonal(self):
 """ Drops self-contacts from the network dataframe. """

        self.network = self.network.where(
            self.network['i'] != self.network['j']).dropna()
        self.network.reset_index(inplace=True, drop=True)