Prevent `int64` to `int` truncation in `Shard` API usage.

The function argument in `Shard` must be a function of two `int64` arguments. However, we are passing in a function with two `int` arguments. Thus, for large workloads, these arguments get truncated from positive `int64` values to negative `int` ones, resulting in a buffer out of bounds write.

PiperOrigin-RevId: 332557334
Change-Id: I236c9a2e7f53580e520571da8ba941a3aa9fa0b5