Understand sentiment book chapter results in validation accuracy of 100%

[sidgoyal78]

The book chapter on sentiment analysis results in a validation accuracy of 100% right in the first epoch. (Thanks to @Sharan for pointing this).

It seems that the error has something to do with the buf_size in the paddle reader: https://github.com/PaddlePaddle/Paddle/blob/develop/python/paddle/fluid/tests/book/test_understand_sentiment.py#L177

On increasing the buf_size to higher values (like 30000), we get reasonable results.

[shanyi15]

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