[optimizer] Refactor optimizers

[optimizer] Refactor optimizers

refactor optimizers to update to the new design. Now there are
two internal optimizer headers -
1. opitmizer_devel - which is used external of the optimizer. The
interface provided here must be sufficient to use the optimizer by
the models or other classes.
2. optimizer_impl - which also follows the same interface but provides
implementations for some more functions as well as some more member
variables.

optimizer.h in the API is a subset of the optimizer_devel which is the
exposed API for an external user to use.
optimizer_devel.h is the API which a user must implement when creating
their own optimizer.
optimizer_impl.h provides another class which a custom optimizer implementer
can extend for their ease.

These classes follow the below design
optimizer.h (ml::train::Optimizer) <- optimizer_devel.h (nntrainer::Optimizer)
<- optimizer_impl.h (nntrainer::OptimizerImpl)

Further, model, graph, and all other classes contains objects of type nntrainer::Optimizer

The c++ API is also reduced to match with the C API.
Once all the c++ APIs are matched with c API, then we can move more functions
from *_devel.h files to *.h files.

See also #986

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

Signed-off-by: Parichay Kapoor <pk.kapoor@samsung.com>