v0.40.8-rc0: mlx: add Flint model support (#19181)

发生了什么

Preserve source tool JSON alongside decoded API values so Flint can retain numeric types and schema order in prompts. Existing API marshaling is unchanged. Assemble ModelOpt expert banks during safetensors import, and reject direct Flint NVFP4 creation before conversion. Other models retain their existing import and quantization paths. Add optional learned sinks and device causal masks to shared MLX attention. Exist…

摘要按规则整理自下方来源原文

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