Fix binary_crossentropy dtypes and slogdet sign on Metal

[MarcosAsh]

This turns main green again after keras pluggable_backend moved to aca7bf905. It fixes two unrelated things:

• binary_crossentropy casts its inputs to a float dtype like torch does (keras#23663). A bool target failed on mlx's negative.
• slogdet divides the real and imaginary parts separately. mlx 0.32.3's Metal complex division gives nan for a tiny divisor, so the sign came out nan on macOS.

They're in one PR because each alone leaves the other leg red.

[hertschuh]

Thanks for the fix!