feat(pnnx): convert prelu[num_parameters=1] to leakyrelu, so that it can be fused with conv

[w43322]

before:

after:

[tencent-adm]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ w43322
❌ nihui
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[github-actions]

The binary size change of libncnn.so (bytes)

architecture	base size	pr size	difference
x86_64	15190272	15159560	-30712 😘
armhf	6196764	6182416	-14348 😘
aarch64	9522968	9454784	-68184 😘

[nihui]

this pass shall be in pass_level5

[w43322]

this pass shall be in pass_level5

done

[w43322]

I'll look at the failed UTs later, 打工人太难了😭

[nihui]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Another round soon, please!

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[Copilot]

Pull request overview

This PR enhances pnnx graph rewriting to improve downstream fusion opportunities by converting scalar nn.PReLU (num_parameters=1) into nn.LeakyReLU, and also refines float/double serialization to better preserve round-trippable numeric strings in generated artifacts.

Changes:

• Add a level5 graph rewrite pass that converts nn.PReLU num_parameters=1 into nn.LeakyReLU with negative_slope taken from the PReLU weight.
• Improve float_to_string() / double_to_string() formatting by detecting non-roundtrippable default %e/%g output and retrying with higher precision, plus normalizing exponent formatting.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
tools/pnnx/src/utils.cpp	Adds exponent normalization and round-trip-aware formatting for float/double string serialization.
tools/pnnx/src/pass_level5/fuse_static_prelu.cpp	Adds a rewrite pass to convert scalar PReLU into LeakyReLU to enable later fusions.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

[nihui]

Thanks for your contribution !