Function elementwise_mul 

pub fn elementwise_mul(
    encoder: &mut CommandEncoder,
    registry: &mut KernelRegistry,
    device: &DeviceRef,
    a: &MlxBuffer,
    b: &MlxBuffer,
    output: &MlxBuffer,
    n_elements: usize,
    dtype: DType,
) -> Result<()>

Encode elementwise multiplication: output = a * b.

Both inputs and output must have the same dtype (F32 or F16).