I caught places where the warning about using grad w.r.t. a one-element
list was raised when running the test-suite.
This commit removes these warnings (ensuring changing the default value of
keep_wrt_type to True will not affect tests).

Note that I changed the behavior of 'test_dot_w_self'. I asked the
author (James) before doing so: it turns out it was not behaving as
intended before.