fix BucketSentenceIter bug related to #11430

[Soonhwan-Kwon]

Description

BucketSentenceIter has bug
when there are buckets with no item.
It keeps invalid buckets as self.buckets
but self.data contains only valid buckets and
it leads to the mismatch with sentence length of symbol and actual data shape.
I fixed the problem by checking valid buckets and handed it to self.buckets.
This fixes issue #11430.

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage:
• To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

[szha]

Would you mind adding a small test for this change?

[Soonhwan-Kwon]

Of course not thank you for the review.

[Soonhwan-Kwon]

I meant you can absolutely feel free to write a small test. I'm sorry if the answer I wrote before confuses you.

[Soonhwan-Kwon]

@szha I added test case , by adding smallest empty bucket, and it reproduces error in previous rnn/io.py version and it works good in patched version.

below is the reproduced error.
Traceback (most recent call last):
File "test_bucketing.py", line 110, in
test_bucket_module()
File "test_bucketing.py", line 102, in test_bucket_module
batch_end_callback=mx.callback.Speedometer(batch_size, 50))
File "/home/ubuntu/anaconda3/envs/mxnet_p27/lib/python2.7/site-packages/mxnet-1.3.0-py2.7.egg/mxnet/module/base_module.py", line 515, in fit
self.forward_backward(data_batch)
File "/home/ubuntu/anaconda3/envs/mxnet_p27/lib/python2.7/site-packages/mxnet-1.3.0-py2.7.egg/mxnet/module/base_module.py", line 194, in forward_backward
self.forward(data_batch, is_train=True)
File "/home/ubuntu/anaconda3/envs/mxnet_p27/lib/python2.7/site-packages/mxnet-1.3.0-py2.7.egg/mxnet/module/bucketing_module.py", line 455, in forward
data_batch.provide_label)
File "/home/ubuntu/anaconda3/envs/mxnet_p27/lib/python2.7/site-packages/mxnet-1.3.0-py2.7.egg/mxnet/module/bucketing_module.py", line 376, in switch_bucket
force_rebind=False, shared_module=self._buckets[self._default_bucket_key])
File "/home/ubuntu/anaconda3/envs/mxnet_p27/lib/python2.7/site-packages/mxnet-1.3.0-py2.7.egg/mxnet/module/module.py", line 430, in bind
state_names=self._state_names)
File "/home/ubuntu/anaconda3/envs/mxnet_p27/lib/python2.7/site-packages/mxnet-1.3.0-py2.7.egg/mxnet/module/executor_group.py", line 279, in init
self.bind_exec(data_shapes, label_shapes, shared_group)
File "/home/ubuntu/anaconda3/envs/mxnet_p27/lib/python2.7/site-packages/mxnet-1.3.0-py2.7.egg/mxnet/module/executor_group.py", line 375, in bind_exec
shared_group))
File "/home/ubuntu/anaconda3/envs/mxnet_p27/lib/python2.7/site-packages/mxnet-1.3.0-py2.7.egg/mxnet/module/executor_group.py", line 662, in bind_ith_exec
shared_buffer=shared_data_arrays, **input_shapes)
File "/home/ubuntu/anaconda3/envs/mxnet_p27/lib/python2.7/site-packages/mxnet-1.3.0-py2.7.egg/mxnet/symbol/symbol.py", line 1528, in simple_bind
raise RuntimeError(error_msg)
RuntimeError: simple_bind error. Arguments:
data: (128, 20L)
softmax_label: (128, 20L)
Error in operator split0: [06:10:03] src/operator/./slice_channel-inl.h:216: Check failed: ishape[real_axis] == static_cast<size_t>(param.num_outputs) (20 vs. 10) If squeeze axis is True, the size of the sliced axis must be the same as num_outputs. Input shape=[128,20,25], axis=1, num_outputs=10.

[Soonhwan-Kwon]

I rebased pr.