In `PrepareForExportPass`, the added broadcast inherits the `kShardingAttr` from the original constant op.
In `PrepareForExportPass`, the added broadcast inherits the `kShardingAttr` from the original constant op.

Before this cl, `kShardingAttr` of the splat constant is dropped in the `PrepareForExportPass`. This cl fixes this issue and adds a test case, which fails without this cl and succeeds with this cl.
