fix: RVV Similarity classes missing simdwidth causes compilation failure on riscv64

[CLiqing]

Description

Compiling knowhere on riscv64 (SG2044 server) fails with:

ScalarQuantizerCodec.h:744:36: error: 'simdwidth' is not a member of 'faiss::SimilarityL2_rvv<0>'
  744 |     constexpr int SIMDWIDTH = Sim::simdwidth;

Root Cause

Commit 55c18bb ("Add fallback and uts for sq4u #1421") added a QT_4bit_uniform case in select_distance_computer_rvv that falls back to the base select_distance_computer. The base function requires Sim::simdwidth, but SimilarityL2_rvv<0> and SimilarityIP_rvv<0> only define metric_type, missing the simdwidth member.

Steps to Reproduce

Build knowhere on riscv64, or cross-compile with riscv64-linux-gnu-g++:

riscv64-linux-gnu-g++-12 -std=c++17 -c ScalarQuantizerDC_rvv.cpp

Expected Behavior

Compilation succeeds on riscv64.

[maxiaohong2333]

On version 2.6.11, I added fix in #1586.
But I still got compile failure :
I work on SG2044 + gcc_14.2.0 + "-std=c++17 -march=rv64gcv_zvfhmin".
It won't happen if I use :
case ScalarQuantizer::QT_4bit_uniform:
return new DCTemplate_rvv<
QuantizerTemplate_rvv<
Codec4bit_rvv,
QuantizerTemplateScaling::UNIFORM,
0>,
Similarity,
0>(dim, trained);

The build error log:
build.txt

[maxiaohong2333]

The build had passed after PR1586, thank you for your help.
https://github.com/zilliztech/knowhere/pull/1586l