need impl num_traits::ApproxEq for half data type

so that we can compare by using the `num_traits::assert_approx_eq` macro

```
assert_approx_eq!(half::f16, self.test_atomic_add_result(input), out);
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `ApproxEq` is not implemented for `f16`
```