Prevent integer truncation from 64 to 32 bits.

The `tensorflow::Shard` functions last argument must be a 2 argument function where both arguments are `int64` (`long long`, 64 bits). However, there are usages where code passes in a function where arguments are `int` or `int32` (32 bits). In these cases, it is possible that the integer truncation would later cause a segfault or other unexpected behavior.

PiperOrigin-RevId: 332560414
Change-Id: Ief649406babc8d4f60b3e7a9d573cbcc5ce5b767