[optimizer] Add optimizer section to INI

[optimizer] Add optimizer section to INI

Add a new optimizer section to INI
This is essential to allow custom optimizers as the
properties of optimizers needs to be iterables.
Currently, optimizer needs to specified in the model, which
along being not the best design, limits the properties to
be set for the optimizer as the model loader has to know
which properties are for optimizer and which for model.

This patch separates the optimizer as a new section and updates
the unittests.

Note that this removes the default optimizer from adam. Now, it is
necessary for an optimizer to be defined in the model file, if to be used for training.

For inference, model can be defined without an optimizer.
Updated documentation for the changes made.

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>