Convenience improvements for llama-quantize

[Nexesenex]

Hey IK.

Here are some ideas of potential features for llama-quantize, that I'm not capable to code myself :

• Create a directory when it doesn't exist for the output file.

• Interrupt the quantization (or even quantize each tensor in a directory, so the quantization can be resumed on crash, or even a single series of tensor can be requantized (like attn_q weight for example, or even a function of use_more_bits if one of the part of the ternary statement deciding the quantization of a given tensor is not met when you change the quant of a part of the ternary, but not the other). The monolithic approach makes a pretty monster-file, but at the same time, wastes a lot of space, time and compute.

• integrate the formulas like use_more_bits (we have one, I intend to PR more of those) to the tensors that we manually specify with arguments in CLI to customize a FTYPE.

• A pre-check of the available space on disk before the quantization, ideally coupled with a dry-run giving the final size of the desired quant.