[WIP] gemm block quantization for llm decoder style

[nihui]

• [ ] move int468 dequant code to x86/arm/... create_pipeline
• [ ] how to encode block_size and storage type ?
• [ ] try fp4 e2m1/e3 type ?
• [ ] comp table ?
• [ ] expand to more general gemm ?
• [ ] port union hack to platform-independent style
• [ ] gemm test++
• [ ] doc++

./ncnnllm2int468 qwen3_decoder.ncnn.param qwen3_decoder.ncnn.bin qwen3_decoder-int6.ncnn.param qwen3_decoder-int6.ncnn.bin

[tencent-adm]

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[codecov-commenter]

Codecov Report

:x: Patch coverage is 7.52688% with 86 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 95.88%. Comparing base (37336e7) to head (58cc1f3). :warning: Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/layer/gemm.cpp	7.52%	86 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6439      +/-   ##
==========================================
+ Coverage   95.62%   95.88%   +0.26%     
==========================================
  Files         844      844              
  Lines      266761   266834      +73     
==========================================
+ Hits       255080   255859     +779     
+ Misses      11681    10975     -706     

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[github-actions[bot]]

The binary size change of libncnn.so (bytes)

architecture	base size	pr size	difference
x86_64	15316400	15324592	+8192 :warning:
armhf	6229892	6234020	+4128 :warning:
aarch64	9527616	9527536	-80 :kissing_heart: