Reduce duplicated function call

Reduce duplicated function call

**Changes proposed in this PR:**
- Reduce `average()` call in `Optimizer::calculate
- Delete setZero call after construction `Tensor`
- Reduce indexing and multiplication in few loops

This PR results in better performance including roughly 60% decrease
in time spent in `Tensor::average()` in
Classification example(checked in vtune).

**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>