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

* gguf-split: split and merge gguf files per tensor

* gguf-split: build with make toolchain

* gguf-split: rename `--split-tensors-size` to `--split-max-tensors`. Set general.split_count KV to all split

* split : minor style + fix compile warnings

* gguf-split: remove --upload not implemented

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Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>