reward_nets: Add easy hidden size configuration (#296)
reward_nets: Add easy hidden size configuration (#296)

Previously, `base_reward_net` and `potential_net` only accepted
`nn.Module`. Now they also accept `Sequence[int]`, which can be
used to build a default MLP with particular hidden sizes.
humancompatibleai/imitation,HumanCompatibleAI/imitation,humancompatibleai/imitation,HumanCompatibleAI/imitation