Add HiddenAct::Silu (remove serde alias)

[alvarobartt]

What does this PR do?

TL;DR Leverage HiddenAct.forward and handle SiLU and SwiGLU separately

This PR fixes the forward method in the HiddenAct implementation to use the SiLU activation when the activation is SwiGLU, since SwiGLU is SiLU + down projection to half the size since it's split on the intermediate dimension (as per https://github.com/huggingface/candle/blob/17313a4226a6c6bde444d28b4be4f0f96d155be7/candle-nn/src/ops.rs#L44-L47), and since candle brings an implementation for it, then the serde alias for Silu has been removed to handle separately both Silu and Swiglu.

Additionally, to unify the codebase, this PR leverages the forward method in HiddenAct instead of the previous match self.act.

Fixes #629

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Who can review?

@kozistr since they recently introduced the forward method in HiddenAct and @Narsil to verify the current patch

[kozistr]

Thanks for the quick fix!

afaik, currently, many models such as Jina, NomicBert, Qwen3 use gated silu activation (== Swiglu) and their codes are like this let gated = self.act.forward(&gated)?;, which is forwarding the linear layer w/ Swiglu activation directly instead of manually splitting in the half and multiply.

Maybe, I guess replacing from Self::Swiglu => candle_nn::ops::swiglu(x) to Self::Swiglu => x.silu() could lead the inconsistency of the model output.

IMHO, how about replacing Swiglu to Silu in HiddenAct enum and implement the gated logic for every model in terms of readability? (And also we can create like a impl GatedLinear to do gated stuff)

please feel free to give me any idea or feedback :)