Optimization: support unsafe_merge.

[omry]

Every config node in Hydra has a reference to the parent (needed for interpolation, among other things).
This means a config node cannot belong to two configs.
Hydra merge APIs do not change the input configs. This means that there is a deepcopy involved.

c1 = OmegaConf.create(...)
c2 = OmegaConf.create(...)
merged = OmegaConf.create(c1, c2)

In most cases, the input configs c1 and c2 above are discarded after the merge, which means the cost of the deepcopy is paid for no good reason.

    @staticmethod
    def unsafe_merge(
        *configs: Union[
            DictConfig,
            ListConfig,
            Dict[DictKeyType, Any],
            List[Any],
            Tuple[Any, ...],
            Any,
        ],
    ) -> Union[ListConfig, DictConfig]:
        """
        Merge a list of previously created configs into a single one
        This is much faster than OmegaConf.merge() as the input configs are not copied.
        However, the input configs must not be used after this operation as will become inconsistent.
        :param configs: Input configs
        :return: the merged config object.
        """