pnnx fix sanitize op from convert.py

[glenn-jocher]

Fix: preserve hyphens in export paths

• Remove the “sanitize” line that strips - from output paths (breaks valid dirs/files).
• Hyphens are valid cross-platform; path mangling surprises users and breaks scripts.

Repro

from ultralytics import YOLO
YOLO("yolo11n.pt").export(format="ncnn", project="runs/export", name="my-model-v1.2")
# before: runs/export/my_model_v1.2/
# after:  runs/export/my-model-v1.2/

Impact

• Paths preserved verbatim; no conversion changes.

Tests

• Add export test to tmpdir/"my-model-v1.2" and assert outputs exist in that exact dir.

Notes

• Reference: @Y-T-G @nihui PR to help Use PNNX Python interface for NCNN conversion ultralytics/ultralytics#22347

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❌ nihui
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[github-actions]

The binary size change of libncnn.so (bytes)

architecture	base size	pr size	difference
x86_64	15208240	15208240	0 😘
armhf	6210540	6210540	0 😘
aarch64	9523592	9523592	0 😘

[nihui]

Because pnnx automatically renames xxx-pnnx.py to xxx_pnnx.py to ensure it can be imported in Python code, I've modified it to keep the folder name part and only replace the hyphens in the .py filename.

[nihui]

Thanks for your contribution !

[glenn-jocher]

Thanks @nihui!

@Y-T-G does this fix the blocker we had using the NCNN python package?