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
What does get_fscore of a ml model in xgboost do?