V.6.0.0 asr_train with transformer raise error
Given that the argument `atype` was moved to e2e_asr.py, the asr.py will raise error in chainer backend when the config is set to train the transformer.

```
Traceback (most recent call last):
  File "/export/db/espnet/egs/voxforge/asr1/../../../espnet/bin/asr_train.py", line 341, in <module>
    main(sys.argv[1:])
  File "/export/db/espnet/egs/voxforge/asr1/../../../espnet/bin/asr_train.py", line 325, in main
    train(args)
  File "/export/db/espnet/espnet/asr/chainer_backend/asr.py", line 81, in train
    if args.atype not in ['noatt', 'dot', 'location']:
AttributeError: 'Namespace' object has no attribute 'atype'
```

not sure if it is required to remove:

```python
    # check attention type
    if args.atype not in ['noatt', 'dot', 'location']:
        raise NotImplementedError('chainer supports only noatt, dot, and location attention.')
```
or not move to another part. Let me know to submit a PR
diff --git a/espnet/asr/chainer_backend/asr.py b/espnet/asr/chainer_backend/asr.py
index f891775a6..371105682 100644
--- a/espnet/asr/chainer_backend/asr.py
+++ b/espnet/asr/chainer_backend/asr.py
@@ -77,10 +77,6 @@ def train(args):
     logging.info('#input dims : ' + str(idim))
     logging.info('#output dims: ' + str(odim))
 
-    # check attention type
-    if args.atype not in ['noatt', 'dot', 'location']:
-        raise NotImplementedError('chainer supports only noatt, dot, and location attention.')
-
     # specify attention, CTC, hybrid mode
     if args.mtlalpha == 1.0:
         mtl_mode = 'ctc'
diff --git a/espnet/nets/chainer_backend/rnn/attentions.py b/espnet/nets/chainer_backend/rnn/attentions.py
index f2a5e848c..fd26e4371 100644
--- a/espnet/nets/chainer_backend/rnn/attentions.py
+++ b/espnet/nets/chainer_backend/rnn/attentions.py
@@ -2,9 +2,7 @@ import chainer
 import chainer.functions as F
 import chainer.links as L
 
-import logging
 import numpy as np
-import sys
 
 from espnet.nets.chainer_backend.nets_utils import linear_tensor
 
@@ -253,7 +251,5 @@ def att_for(args):
     elif args.atype == 'noatt':
         att = NoAtt()
     else:
-        logging.error(
-            "Error: need to specify an appropriate attention architecture")
-        sys.exit()
+        raise NotImplementedError('chainer supports only noatt, dot, and location attention.')
     return att
['espnet/nets/chainer_backend/rnn/attentions.py', 'espnet/asr/chainer_backend/asr.py']