Skip missing expert bias updates

[yueming-yuan]

Summary

• Skip router expert-bias updates for modules whose expert_bias is absent or None
• Assert local_tokens_per_expert exists for modules that participate in expert-bias updates

Validation

• Covered by the DeepSeek V4 16-node disaggregate debug run that reached step 1 with this change in place
• No local test suite run for this single-file Megatron change