Minor revision about example scripts

[Minys233]

I noticed in example code, some argument are inconsistent in the commented parts, such as lines in examples/molnet/train_molnet.py: https://github.com/chainer/chainer-chemistry/blob/56e83dedb5de9dc9eb08ebf292be9ba76a4883ba/examples/molnet/train_molnet.py#L267-L274

args.gpu and concat_mols are not consistent with other codes and will throw errors. They should be modified to device and converter, since they are extracted from args or defined in previous lines like this: https://github.com/chainer/chainer-chemistry/blob/56e83dedb5de9dc9eb08ebf292be9ba76a4883ba/examples/molnet/train_molnet.py#L209 https://github.com/chainer/chainer-chemistry/blob/56e83dedb5de9dc9eb08ebf292be9ba76a4883ba/examples/molnet/train_molnet.py#L233

Since this mistake is too minor to PR, a small issue is enough. Have a nice day :-)

[corochann]

Sorry for confusion for remaining the old legacy code, and thank you for report.