Cosmetic change to type variable declarations in base
Cosmetic change to type variable declarations in base

PEP-484 is not very clear about the restrictions on type variable declarations,
but mypy seems to reject multi-assignments, e.g.

  T, U = TypeVar("T"), TypeVar("U")

PiperOrigin-RevId: 467263968

deepmind/dm-haiku,deepmind/dm-haiku