fix: preserve torch_dtype in get_engine_config and add it to CodeFormulaV2 (#3117)

* fix: preserve torch_dtype in get_engine_config and add it to CodeFormulaV2

get_engine_config() was silently dropping torch_dtype from engine
overrides when constructing EngineModelConfig. This caused
CodeFormulaV2 to load in fp32 while using Flash Attention 2, which
only supports fp16/bf16.

Also adds torch_dtype="bfloat16" to the CodeFormulaV2 preset's
TRANSFORMERS engine override.

Fixes #3026

Signed-off-by: majiayu000 <1835304752@qq.com>

* fix: move torch_dtype into extra_config per review feedback

Signed-off-by: majiayu000 <1835304752@qq.com>

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Signed-off-by: majiayu000 <1835304752@qq.com>