#tf-data Fix global shuffling for shard dataset.
#tf-data Fix global shuffling for shard dataset.

The issue is similar to batching without dropping remainders:
The input dataset may return `end_of_sequence` in the middle
of its output since the index is shuffled.

For example, without fix, when shuffling
```
range(10).shard(3, 0)
```
The range dataset could return `end_of_sequence` after producing
9.

This CL fixes the index mapper for `shard` so no skipping is
needed for global shuffling.
