get_rows & dequantize function implementation for repacked weights of type q6_K (q6_Kx8)

[swetha097]

NOTE: Creating the PR changes required for whisper.cpp here as llama.cpp already includes test-backend-op

• This implements the GGML_OP_GET_ROWS operation specifically for repacked (block interleaved) 6-bit quantized format (q6_Kx8).
• The following gains were observed by the changes made in the PR - The changes allow for increased usage of the GEMM function (ggml_gemm_q6_K_8x8_q8_0) for q6_K type.
• The PR was tested in AMD Raphael 7600X for whisper - which supports the following flags :
  system_info: n_threads = 4 / 12 | WHISPER : COREML = 0 | OPENVINO = 0 | CPU : SSE3 = 1 | SSSE3 = 1 | AVX = 1 | AVX2 = 1 | F16C = 1 | FMA = 1 | BMI2 = 1 | AVX512 = 1 | AVX512_VBMI = 1 | AVX512_VNNI = 1 | AVX512_BF16 = 1 | LLAMAFILE = 1 | OPENMP = 1 | AARCH64_REPACK = 1 |

master branch commit - swetha097/whisper.cpp@fc45bb8

q6K repacking commit ( block interleaving approach for Q6_K quantization for x64/x86 SIMD Architecture )- swetha097/whisper.cpp@d89aaf2

development (get_rows) branch commit - swetha097/whisper.cpp@de9839e

Model for performance tests Downloaded from : https://huggingface.co/ggerganov/whisper.cpp/blob/main/ggml-base.en.bin and quantized to q6_K

This patch of the code was also tested with llama.cpp repository & the perplexity of Q6_K models were ensured to be the same before and after the changes made :

Final estimate: PPL = 5.3669 +/- 0.13305

Model used for perplexity test quantized from - https://huggingface.co/meta-llama/Llama-2-7b

This PR is to merged after - Q6_K - Block Interleaving Implementation for x86 SIMD (AVX512/AVX2) #15275