fix eval for moe layer

[NeosZhang]

when I run /exp/dmoe/dmoe_46m_8gpu.sh，I encountered the following Error during evaluation.

the func save_load_balancing_loss is bypassed during evaluation.

[mvpatel2000]

@NeosZhang is there a reason you are calling batched loss during eval? We explicitly do not store routing stats as it would affect the next update after an eval. Instead, the recommended solution is to avoid calling the loss fn here.

We can probably give a friendlier error here though... CC: @eitanturok

[eitanturok]

@mvpatel2000 , I took a look at the exp/dmoe/dmoe_46m_8gpu.sh script and it seems like we may need to modify the arguments used there to not get the error.