[nnkit] Construct CaffeBackend from caffe::Net (#967)

[nnkit] Construct CaffeBackend from caffe::Net (#967)

* [nnkit] Construct CaffeBackend from caffe::Net

Currently, the constructor of CaffeBackend takes arguments for
caffe::Net, and forwards it to internally allocated caffe::Net instance.

This commit revises CaffeBackend constructor to take an exteranlly
allocated caffe::Net instance as its argument.

This change allows us to construct a caffe backend from in-memory caffe
model (instead of serialized one).

Signed-off-by: Jonghyun Park <jh1302.park@samsung.com>
* Remove unnecessary comment