Fix API docs: use math.pi rather than np.pi in keyword arg default value.
Fix API docs: use math.pi rather than np.pi in keyword arg default value.

Context: Our automatic API generation at readthedocs uses sphinx autodoc, which
relies on importing Python modules en route to generating the docs. Numpy is
not part of the excecution environment there, so it is flagged in the autodoc
config and mocked out during the generation process. Default arg values are
executed at function definition time -- during imports -- hence cannot contain
Numpy code. Nice little gotcha!
PiperOrigin-RevId: 368120427

google/trax,google/trax