ensure str for the case of bytes type.

[jnory]

Hi,

I noticed that create_pretraining_data.py aborts by the error:

  File "albert/create_pretraining_data.py", line 405, in <listcomp>
    for i in piece])):
AttributeError: 'int' object has no attribute 'lower'

The reason why the error occurs is that the variable piece may be a bytes type in Python 3.

I'm using sentencepiece tokenizer, and, the minimal case of the input text is following (the text comes from wikipedia):

カムデンは（39.937195, -75.106186）に位置する。

This small PR fixes the problem by ensuring str type for the piece.
Please let me know if you notice anything.

Sincerely,

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[jnory]

@googlebot I signed it!

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