This PR simply replicates the tensor per tensor custom quantization CLI feature brought by Ikawrakow for the token embeddings and output tensors in ggerganov#6239 to :
- attn_q.weight
- attn_k.weight
- attn_v.weight
- attn_qkv.weight
- attn_output.weight
- ffn_gate
- ffn_down
- ffn_up

This, to allow LlamaCPP users to easily tailor their chosen quant strategy to their needs, but ALSO to allow them to requant easily a quant "a bit too big" for their VRAM in the case of GPU users.

For example, a nice Miqu 70b Q5_K_M (which has no FP16 weight available beyond dequants of Q5_K_M) is short of VRAM in one's pair of 3090s.
And one is French, like me, so Miqu is one of his main local model.

Requanting the Q5_K_M in... Q5_K_M, BUT with all the ffn_down and attn_v.weight tensors specified in Q5_K, and the attn_q.weight specified in Q4_K_M might save you approximatively 1.5GB without degrading too much the quality.
That means 1.3-1.4GB of additional context (yummy with FA and KV Cache) and let's say 100-200MB of additional compute cache with a resonable Blas Batch Size in MMQ.

But also : the unspecified tensors won't be requantized, because LlamaCPP just copy the tensor rather than requantizing it when a specific tensor quant of the chosent strategy is the same than the source.
So one can enjoy the original Miqu quant of these tensors rather than a dequant/requant.

And that's just an example.

I think that many LCPP users could enjoy this feature for their own needs.

This, even if it remains quite basic :
This PR doesn't support hybrid quantization of a tensor (example, with a fraction of the layers in the upper quant (from layer 0 onwards), or the "more_bits" calculus devised by Ikawrakow to create intervals of different quants (ex : 1 layer every 3 layers quantized with the superior quant).

CL example: `llama-quantize --allow-requantize --imatrix Q:\iMatrix\Sheared\princeton-nlp_Sheared-LLaMA-2.7B-AR-b1924-Q8_0.iMatrix_Wiki_c32_ch500.dat --output-tensor-type q4_0 --token-embedding-type q4_0 --attn-q-type q4_0 --attn-k-type q4_0 --attn-v-type q4_0 --attn-output-type q4_0 --ffn-gate-type q4_0 --ffn-down-type q4_0 --ffn-up-type q4_0 D:\text-generation-webui\models\Q8_0\princeton-nlp_Sheared-LLaMA-2.7B-AR-b1924-Q8_0.gguf D:\text-generation-webui\models\princeton-nlp_Sheared-LLaMA-2.7B-AR-b228N.iMatrix_Wiki_c32_ch500-Q5_K_M.gguf Q5_K_M` for a full q4_0 quant equivalent to a pure quant, but specified tensor by tensor.

And integrate it in the tensors quantization tree.