[Tensor] OutputGrad defaults to be zero if not given

[Tensor] OutputGrad defaults to be zero if not given

This patch creates output grad(incoming) to all zero if the given output
is not trainable. This will make the given output be considered as
constant. If a user wants to check if output is constant-like(having
zero gradient as it's partner), she can easily check if
outputHasGradient

**Self evaluation:**
1. Build test: [X]Passed [ ]Failed [ ]Skipped
2. Run test: [X]Passed [ ]Failed [ ]Skipped

Signed-off-by: Jihoon Lee <jhoon.it.lee@samsung.com>