Get feature importance of each feature.

        Parameters
        ----------
        fmap: str (optional)
           The name of feature map file

def get_fscore(self, fmap=''):
        """Get feature importance of each feature.

        Parameters
        ----------
        fmap: str (optional)
           The name of feature map file
        """
        trees = self.get_dump(fmap)
        fmap = {}
        for tree in trees:
            for line in tree.split('\n'):
                arr = line.split('[')
                if len(arr) == 1:
                    continue
                fid = arr[1].split(']')[0]
                fid = fid.split('<')[0]
                if fid not in fmap:
                    fmap[fid] = 1
                else:
                    fmap[fid] += 1
        return fmap