Guard zero-sum MoE gate normalization

[WangXukang-cypher]

Summary

This PR adds epsilon guards to MoE gate normalization paths to avoid NaNs when the selected gate weights sum to zero.

Specifically, it updates the normalization logic in:

• TopKGating
• TopKDropFreeGating
• TopKBiasGating

It also adds regression tests covering zero-score / zero-sum gate cases, including the bias-gating group-routing path.

Test Plan

• Ran 27 existing MoE tests: all passed
• Observed 1 failing test: test_moe_quantized_einsum0
  • This is a pre-existing TPU-only quantized test that fails on CPU
  • It is unrelated to this epsilon fix
• Ran targeted regression tests for this change in the local conda axlearn environment: 5 passed
  • TopKGatingTest::test_sigmoid_score_fn
  • TopKGatingTest::test_zero_score_fn_produces_finite_combine_tensor
  • TopKDropFreeGatingTest
  • TopKBiasGatingTest