Change loading meta information behavior

Change loading meta information behavior

**Before this PR**

optimizer variable loaded from load_path every time.
Calling model->train(); in a row became unintuitive

1. model->train() load from original load path
thus iteration number roll back to the first one.
2. Same happens for the adam weight
3. model->load(); after model->initialize(); is noop
because loadedWeight becomes true

**After this PR**

1. model load from load_path only at initialize time
2. model->load is not implicitly overriden

**Additional Changes**

1. optimizer weight became part of weights. Now available after initialize()
2. Save format became coherent with load format
3. Some unused variables deleted

**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>