split: allow --split-max-size option

[phymbert]

Motivation

we support --split-max-tensors since:

• gguf-split: split and merge gguf per batch of tensors #6135

As mentionned by @Artefact2 in this comment:

allowing to split by file size would be more intuitive (and usually more appropriate since file size is usually the limiting factor, eg 4G for FAT or 50G for HF)

Proposition:

Introduce --split-max-size N(M|G) split strategy to split files in file with a max size of N Megabytes or Gigabytes.
As it is not possible to have less than 1 tensor per GGUF, this size is a soft limit.

[phymbert]

Tests are missing, and we need to show how to use the gguf-split feature, it should be integrated in the CI. This is mandatory as shard feature is more and more used on HF.