Make the dummy batch finish with done True

This can be used to support algorithm or postprocessing than only works on complete_episodes. I use for myself a wrapper:

def _get_dummy_batch_from_view_requirements(
    self, batch_size: int = 1
) -> SampleBatch:
    dummy_sample_batch = super()._get_dummy_batch_from_view_requirements(
    dummy_sample_batch[dummy_sample_batch.DONES][-1] = True
    return dummy_sample_batch

This is great @Maxime_Riche! Could you do a PR? Then we can check, whether all tests pass and make this the norm.