update pnnx torch-2.11

[nihui]

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[Copilot]

Pull request overview

Updates pnnx to be compatible with Torch 2.11 by handling bias-less nn.MultiheadAttention exports more robustly, and refreshes the pnnx CI workflow to build/test against newer Torch/TorchVision/ONNXRuntime versions.

Changes:

• Update nn_MultiheadAttention NCNN pass to always split in_proj_weight, and to synthesize zero biases when bias=False (including out_proj.bias).
• Bump pnnx workflow build dependencies to LibTorch 2.11 / TorchVision 0.26 and ONNXRuntime 1.25.1.
• Extend CI test matrix with a Torch 2.11 / Python 3.14 configuration.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tools/pnnx/src/pass_ncnn/nn_MultiheadAttention.cpp	Makes MultiheadAttention weight/bias handling tolerant of bias=False by defaulting to zero bias and avoiding reads of missing/empty bias attrs.
.github/workflows/pnnx.yml	Updates pinned dependency versions and adds a Torch 2.11 test entry to CI.

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[nihui]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Already looking forward to the next diff.

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