Fix model loading with OptimizerWrapper

[hasan-yaman]

Problem:

• 1f2a9dc tries to load optimizer and LR scheduler. With this change model saved on the previous versions cannot be loaded with the latest version.

Minimum reproducible example:

# git checkout bb984031fef4a9cc92a321aa8ceae3c0bbb64419
from d3rlpy.algos import DiscreteBCConfig
from d3rlpy.datasets import get_cartpole

dataset, _ = get_cartpole()

model = DiscreteBCConfig().create()

model.fit(
    dataset=dataset,
    n_steps_per_epoch=100,
    n_steps=300
)

model.save("bc.d3")

then try to load model

# git checkout 1f2a9dc8ec95a8b295b68497ea290cc00c362b05
# or to the latest commit git checkout 5400c22d812b2dab9ae66b2def28e5b135c354fd
from d3rlpy import load_learnable

model = load_learnable("bc.d3")

It throws KeyError: 'optim'

[takuseno]

LGTM. That's a reasonable solution. Thank you for your contribution!