data downloading for mnist

[zlheui]

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[codecov]

Codecov Report

Merging #851 (ee48750) into dev (4dab111) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #851   +/-   ##
=======================================
  Coverage   60.89%   60.89%           
=======================================
  Files          85       85           
  Lines        4513     4513           
=======================================
  Hits         2748     2748           
  Misses       1765     1765           

Flag	Coverage Δ
singa-cpp	60.89% <ø> (ø)

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