implement flip layer and pnnx torch.flip conversion

[nihui]

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[tencent-adm]

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❌ Baiyuetribe
❌ nihui
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[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.22034% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.96%. Comparing base (e207b3b) to head (4871990).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/layer/flip.cpp	93.22%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6233      +/-   ##
==========================================
+ Coverage   95.91%   95.96%   +0.04%     
==========================================
  Files         835      836       +1     
  Lines      264420   264401      -19     
==========================================
+ Hits       253631   253738     +107     
+ Misses      10789    10663     -126     

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[github-actions]

The binary size change of libncnn.so (bytes)

architecture	base size	pr size	difference
x86_64	15643128	15648008	+4880 ⚠️
armhf	6648156	6649020	+864 ⚠️
aarch64	9987296	9987736	+440 ⚠️

[Copilot]

Pull Request Overview

This PR implements the flip layer functionality for ncnn and PNNX, adding support for torch.flip operation conversion. The implementation includes the core flip layer in ncnn, PNNX conversion passes for both TorchScript and ONNX models, and comprehensive test coverage.

Key changes:

• Added a new Flip layer to ncnn that reverses tensor dimensions along specified axes
• Implemented PNNX passes to convert torch.flip operations from TorchScript and ONNX formats
• Added comprehensive test suites for the flip functionality across different tensor dimensions

Reviewed Changes

Copilot reviewed 20 out of 20 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/layer/flip.h	Header file defining the Flip layer class interface
src/layer/flip.cpp	Core implementation of the Flip layer with forward pass logic
tools/pnnx/src/pass_level2/torch_flip.cpp	PNNX pass for converting torch.flip operations from TorchScript and ONNX
tools/pnnx/src/pass_ncnn/torch_flip.cpp	PNNX pass for generating ncnn-compatible flip operations
tests/test_flip.cpp	Comprehensive test suite for the flip layer functionality
Multiple test files	Test scripts for validating torch.flip conversion in different contexts
Multiple CMakeLists.txt	Build configuration updates to include new flip functionality
Documentation	Updated operator documentation to include flip layer