reduce the number of shared expert streams

[yangbofun]

What does this PR do ?

1. Reduce the number of the share expert stream when using --moe-shared-expert-overlap true.

before:

after:

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[Victarry]

/ok to test fcb68bd

[ericharper]

/ok to test 56e37da

[svcnvidia-nemo-ci]

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You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/24336512931