fix multi worker dataloader deadlock

[piiswrong]

Description

(Brief description on what this PR is about)

Checklist

Essentials

• Passed code style checking (make lint)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage:
• Unit tests are added for small changes to verify correctness (e.g. adding a new operator)
• Nightly tests are added for complicated/long-running ones (e.g. changing distributed kvstore)
• Build tests will be added for build configuration changes (e.g. adding a new build option with NCCL)
• Code is well-documented:
• For user-facing API changes, API doc string has been updated.
• For new C++ functions in header files, their functionalities and arguments are documented.
• For new examples, README.md is added to explain the what the example does, the source of the dataset, expected performance on test set and reference to the original paper if applicable
• To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

Changes

• Feature1, tests, (and when applicable, API doc)
• Feature2, tests, (and when applicable, API doc)

Comments

• If this change is a backward incompatible change, why must this change be made.
• Interesting edge cases to note here

[szha]

LGTM

[fpshuang]

Sry for my poor English translation skills, I'd write this in Chinese.

大佬，我在运行你的天池比赛样例的时候，在第十三个代码块中，运行时发生错误：local variable 'idx' referenced before assignment
错误提示如下
/usr/lib/python2.7/site-packages/mxnet/gluon/data/dataloader.pyc in iter(self)
217 for idx, batch in enumerate(self._batch_sampler):
218 key_queue.put((idx, batch))
--> 219 num_batches = idx + 1
220
221 data_buffer = {}

私以为num_batches = idx + 1应该tab间隔一下，以便作为for loop中进行操作，希望得到大佬的解答。