如何将多键字典转换为pandas数据帧，其中每个键和值都有自己的列？

import pandas as pd

data = {'key1': {'subkey1': value1, 'subkey2': value2},
        'key2': {'subkey1': value3, 'subkey2': value4}}

df = pd.DataFrame.from_dict({(i,j): data[i][j] 
                             for i in data.keys() 
                             for j in data[i].keys()},
                            orient='index')

df = df.reset_index()
df.columns = ['key', 'subkey', 'value']