Fix crash when `tf.pad` is used with int64 paddings. (#13517)

* Add int64 bounds support for `tf.image.pad_to_bounding_box`

This fix tries to fix the issue raised in 13506 where int64 data types
for bounds in `tf.image.pad_to_bounding_box` crashes.

The reason of the crash is caused by the fact that int64 was directly
converted into int32 without passing through kernel registeration.

This fix fixes the issue by adding `typename Tpadding` to the template.