Fix MegatronLayerPolicy to have megatron_v2=True (#2579)
This PR updates the MegatronLayerPolicy to set megatron_v2=True, which is required in order to properly transpose in the replace_with_policy() function.
After the change in this PR, in conjunction with PR #99 in the Megatron-DeepSpeed fork, the Megatron text-generation example works with DS inference.
