Fix: mean shape in compatible with input shape

This commit is to fix issue #5718.

* reference:
1. https://groups.google.com/forum/#!topic/caffe-users/nBpWJCcJoCU
2. https://stackoverflow.com/questions/28692209/using-gpu-despite-setting-cpu-only-yielding-unexpected-keyword-argument

Signed-off-by: Geunsik Lim <geunsik.lim@samsung.com>