Allow mnist data to be writen to remote file system.

[qlzh727]

For b/77657597. Btw, I don't have a TPU instance to validate this change, I think the tf.gfile should do the trick with internal env.

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[googlebot]

CLAs look good, thanks!

[robieta]

Is this sufficient? You're still relying on urllib to be able to write the archive to the file system.

[saeta]

I think this should work. :-)

[qlzh727]

@robieta, the urllib is only used to read the input file object, which does not change within this PR. The tf.gfile.Open should support different file system name space (eg, gs:/ in this case), whereas the default python open() only support local file system. You can read internal doc about gfile API to get more details.