FIX Bug that prevents BOFT from loading 2 adapters (#2068)
There was a bug in BOFT that made it impossible in some circumstances to
load more than one adapter (creating more than 1 adapter was possible
though). This was because a code path that adjusts
boft_n_butterfly_factor was only visited when creating a fresh adapter,
but not when updating with the 2nd adapter. This was fixed by moving
this code path from the BOFT layer's __init__ method to update_layer.
A test for loading multiple adapters was added. Since this was a gap in
our test suite, this test will be applied to all appropriate PEFT
methods, not only BOFT, but the others methods are all passing without
needing further changes.
For good measure, I also added BOFT to the test suite that checks
multiple active adapters. These tests would have also passed without the
fix in this PR, since these tests do not load multiple adapters but
instead create them, which always worked. Still it's better to have
these tests as well.
