Add float16 inference option for TFLite benchmark app
Hi @terryheo Can you please review this PR ? Thank you!
## Background
Native float16 inference is introduced in [Tensorflow Blog](https://blog.tensorflow.org/2023/11/half-precision-inference-doubles-on-device-inference-performance.html).
To try this on benchmark app(benchmark_model), we need set flag of XNNPackDelegateOptions,  it's not accessible.
This PR enable to access the flag for float16 inference from command line args.

## Change
- Add `--xnnpack_force_fp16` option in benchmark_model
  ```bash
  benchmark_model --graph=... \
    --use_xnnpack=true \
    --xnnpack_force_fp16=true # enable float16 inference.
  ``` 