bug: Numeric divergence between the mHC layer and its Pallas kernel

### Bug report

DeepSeek v4. The reference path had its own mapping helper that cast alpha_scale and
beta down to self.dtype before computing sigmoid(alpha_scale * h + beta),
while the Pallas path called common.compute_sigmoid_gate, which keeps the
scale and bias in float32. With dtype: bfloat16 the two branches therefore
gated on different numbers, so toggling use_mhc_pallas_kernel changed the
layer's output beyond accumulation noise.
Fixed in PR:
https://github.com/AI-Hypercomputer/maxtext/pull/5179

### Logs/Output

_No response_

### Environment Information

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### Additional Context

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