 - Re-implement `_ConcatWithPadding` to use `lax.{map, dynamic_update_slice}`.  - Also get rid of `np.roll` from `_StripFromConcatenateWithPadding` and use `map` and `dynamic_slice` there as well.
 - Re-implement `_ConcatWithPadding` to use `lax.{map, dynamic_update_slice}`.
 - Also get rid of `np.roll` from `_StripFromConcatenateWithPadding` and use `map` and `dynamic_slice` there as well.

PiperOrigin-RevId: 315039514

google/trax,google/trax