Add balance loss in atorch moe example

Add balance loss in [moe example](https://github.com/intelligent-machine-learning/dlrover/tree/master/atorch/examples/moe).

We add use aux loss to router in moe example, similar to aux loss implementation in[ megatron moe](https://github.com/NVIDIA/Megatron-LM/blob/0d89fc4c0d4394f915fffff11212d6957652337f/megatron/core/transformer/moe/router.py#L142
).

Start in atorch/examples/moe and inspect how the router currently handles expert selection. Compare it with the linked Megatron MoE router implementation, focusing on its auxiliary balance-loss behavior. Done means the example uses an auxiliary loss in the router to improve expert load balance.

Written by the indexing model from the issue text.