[MoE] Add a new score function to the router

[yaox12]

What does this PR do ?

1. Add a new score function sqrtsoftplus to the router.
2. Change a little to how we handle the precision in the router

• TLDR: In topk_routing_with_score_function, we only convert to BF16 before returning (not in the middle).
• Basic rules:
  • Logits are converted to fp32 at the beginning for score functions.
  • All the intermediate calculations are in fp32.
  • The final probs are casted to the same dtype as the logits.
  • In compute_routing_scores_for_aux_loss, we always use FP32.

Contribution process

flowchart LR
    A[Pre-checks] --> B[PR Tests]
    subgraph Code Review/Approval
        C1[Expert Review] --> C2[Final Review]
    end
    B --> C1
    C2 --> D[Merge]

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[yaox12]

/ok to test 37c8610