Reduce the compilation time of strided_slice

- Break strided_slice op into multiple translation units by dimension
- Refactor code to allow instantiation from multiple translation units
  by using a free function instead of a class member
- strided_slice_op.cc used to take 163-180s. Now the aggregate of all
  translation units is 196s but each strided_slice_op_inst_* takes
  between 24-34s each
- cast to a canonical type for each POD size to avoid redundant
  instantiations for say int32 and float32.
Change: 127578995