[Go] Fix `size` attribute jittable

[sotetsuk]

Currently, this raises error:

@jax.jit
def f():
    return Game(size=9).init().size

print(f())

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (8575a3f) 88.10% compared to head (4ea7189) 88.36%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1158 +/- ## ========================================== + Coverage 88.10% 88.36% +0.25% ========================================== Files 64 64 Lines 5996 5997 +1 ========================================== + Hits 5283 5299 +16 + Misses 713 698 -15 ```

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