[DEV] fix(megatron-fsdp): build expt_device_mesh only for MoE models

[xuwchen]

What does this PR do ?

main PR: #3831

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[shjwudp]

/ok to test 7d6e862

[xuwchen]

/ok to test e75359f

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/23158792959

[yaox12]

/ok to test 8279788

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/23220933852