Fix conversion of unnormalized BF16->BF16 weights (#7843)

Fix conversion of unnormalized BF16->BF16 weights (#7843)

* add truncate_bf16

* truncate intermediate fp32 if converting bf16 to bf16

* fix masking in __compute_fp32_to_bf16

* np.int16 no longer used

* missing cast and additional numpy 2.x fix

* ggml-impl : do not flush bf16 subnormals to zero

* ggml : add reference fp32 to bf16 conversion

The fast version is no longer equivalent for all platforms
because of the handling of subnormal values.

* gguf-py : remove flush to zero for bf16 subnormals

* gguf-py : remove float32 truncation to bf16

Rounding achieves the same thing in the cases where this was used.

* missed prototype update in merge

* merge cleanup

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Co-authored-by: Francis Couture-Harpin <redacted>