Fix pfft gradients

[ASKabalan]

Updating jaxDecomp version and adding tests to ensure that we can get decent gradients

[ASKabalan]

@EiffL

Fell free to bump version and make release if it is urgent
I think that even with jaxdecomp v0.2.2 as dependency the pip installer will select v0.2.3 since the constraint is >=0.2.2

Anyways if tests pass this can be merged

note: BackSolveAdjoint tests are skipped. they won't work because I need to implement JVP custom rules in jaxDecomp instead of VJP (will be done soon)

note2: I think that there is a small modification to do in jaxDecomp to enable BackSolveAdjoint

[EiffL]

Thanks Wassim!