Kumaraswamy distribution bug fixes

[e-pet]

Two small fixes as discussed in #1671.

(I was unable to get the tests to run locally - I'm mainly working on windows, which complicates things a bit. Let me know if this is a serious issue, then I'll try to get them to run.)

[fehiepsi]

Thanks, @e-pet!

[martinjankowiak]

looks like this merge may be resulting in some test failures?

FAILED test/test_distributions.py::test_vmap_dist[Kumaraswamy-None-params47] - AttributeError: 'Kumaraswamy' object has no attribute 'base_dist'
FAILED test/test_distributions.py::test_vmap_dist[Kumaraswamy-None-params48] - AttributeError: 'Kumaraswamy' object has no attribute 'base_dist'
FAILED test/test_distributions.py::test_vmap_dist[Kumaraswamy-None-params49] - AttributeError: 'Kumaraswamy' object has no attribute 'base_dist'

[e-pet]

@martinjankowiak Sorry, I am out of my depth for this and will have to refer to @fehiepsi... Happy to do implement / commit any fixes though, if someone can tell me what I have to do.

[martinjankowiak]

@e-pet no worries this has been dealt with elsewhere !