Return early in hk.dropout if drop rate is 0.
Return early in hk.dropout if drop rate is 0.

Using drop rate = 0 is a common pattern for when the model is in inference mode.

PiperOrigin-RevId: 313072024
Change-Id: I58cd53cb8b6d77b8415128ca60d6a5b28b322bd0

deepmind/dm-haiku,deepmind/dm-haiku