[flex/linen]: Make `rngs` type "Optional" to avoid pytype errors.
[flex/linen]: Make `rngs` type "Optional" to avoid pytype errors.

Setting `rngs` to None in apply() is valid (also the default), but setting it
explicitly in a callee site causes pytype errors.

PiperOrigin-RevId: 371438792

google/flax,google/flax