Mamba model is broken with `f16` precision

[jorgeantonio21]

Running the command:

cargo run --example mamba --release --features metal -- --prompt "Tell me a joke please" --dtype f16

does not work. The problem seems to lie in code:

for &t in tokens.iter() {
            let input = Tensor::new(&[t], &self.device)?;
            let logits = self.model.forward(&input, &mut state)?;
            next_logits = Some(logits);
            if let Some(t) = self.tokenizer.next_token(t)? {
                print!("{t}")
            }
        }

where logits is a Tensor of null values.