matplotlib.use('Agg') fail
It result in plot fail 
```
_tkinter.TclError: no display name and no $DISPLAY environment variable
```
I fixed this by applying a patch on `espnet/nets/pytorch_backend/transformer/plot.py`
```
@@ -1,5 +1,6 @@
 import logging
-
+import matplotlib
+matplotlib.use('Agg')
 import matplotlib.pyplot as plt

 from espnet.asr import asr_utils
```
diff --git a/espnet/nets/pytorch_backend/transformer/plot.py b/espnet/nets/pytorch_backend/transformer/plot.py
index 66985df55..af5f0ef6e 100644
--- a/espnet/nets/pytorch_backend/transformer/plot.py
+++ b/espnet/nets/pytorch_backend/transformer/plot.py
@@ -1,8 +1,7 @@
 import logging
 
-import matplotlib.pyplot as plt
-
 from espnet.asr import asr_utils
+import matplotlib.pyplot as plt
 
 
 def _plot_and_save_attention(att_w, filename):
['espnet/nets/pytorch_backend/transformer/plot.py']