How to save a Seaborn plot into a file

I tried the following code (test_seaborn.py):

import matplotlib
matplotlib.use('Agg')
import matplotlib.pyplot as plt
matplotlib.style.use('ggplot')
import seaborn as sns
sns.set()
df = sns.load_dataset('iris')
sns_plot = sns.pairplot(df, hue='species', size=2.5)
fig = sns_plot.get_figure()
fig.savefig("output.png")
#sns.plt.show()

But I get this error:

  Traceback (most recent call last):
  File "test_searborn.py", line 11, in <module>
    fig = sns_plot.get_figure()
AttributeError: 'PairGrid' object has no attribute 'get_figure'

I expect the final output.png will exist and look like this:

How can I resolve the problem?