the current chainer-mask-rcnn is unadaptable to the latest chainer version

[himlen1990]

Failures occurred when running train.py in the example/custom_dataset/
I had tried some chainer versions (7.7.0 and 6.6.0) and failed.
(the error in 6.6.0 )
Exception in main training loop: Extension implementation must override call.
Traceback (most recent call last):
File "/home/pr2/anaconda2/envs/chainer-mrcnn/lib/python3.6/site-packages/chainer/training/trainer.py", line 319, in run
entry.extension(self)
File "/home/pr2/anaconda2/envs/chainer-mrcnn/lib/python3.6/site-packages/chainer/training/extension.py", line 65, in call
'Extension implementation must override call.')
Will finalize trainer extensions and updater before reraising the exception.
Traceback (most recent call last):
File "train.py", line 93, in
main()
File "train.py", line 88, in main
evaluator_type='voc',
File "/home/pr2/chainer-mask-rcnn/examples/custom_dataset/../train_common.py", line 354, in train
trainer.run()
File "/home/pr2/anaconda2/envs/chainer-mrcnn/lib/python3.6/site-packages/chainer/training/trainer.py", line 349, in run
six.reraise(*exc_info)
File "/home/pr2/anaconda2/envs/chainer-mrcnn/lib/python3.6/site-packages/six.py", line 703, in reraise
raise value
File "/home/pr2/anaconda2/envs/chainer-mrcnn/lib/python3.6/site-packages/chainer/training/trainer.py", line 319, in run
entry.extension(self)
File "/home/pr2/anaconda2/envs/chainer-mrcnn/lib/python3.6/site-packages/chainer/training/extension.py", line 65, in call
'Extension implementation must override call.')
NotImplementedError: Extension implementation must override call.

Finally I installed chainer 4.0.0 and it worked. It seems the current chainer-mask-rcnn is not support chainer version higher than 5.0.0, I think you'd better to modify the requirements.txt.

[knorth55]

@himlen1990 can you try this PR? wkentaro/fcn#121
this PR fix the same issues occurred in my env.