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This change moves back to converting bf16 vision weights to fp16,
specifically if they start with the name "v." (such as v.blk.0.attn_k.weight).

This fixes a bug where converted images are failing because they are trying
to call `im2col` which doesn't have a bf16 kernel in ggml.

if there's no transformation to the tensor and the input and output
types match, copy directly into the writer. also read from a bufio with
a 32K buffer