[Suggestion] : Remove strict requirement on backends.

[ASKabalan]

Currently, we have to install pytorch to use the JAX backend of s2fft.

I think it would be a nice to be able to conditionally activate backends depending on the availability of the package

For example I user can use s2FFT using only numpy if he does not have JAX or pytorch Otherwise he gets a runtime error instead of an import time error

Same for pyssht. Don't know the best practice to do this, but a try catch around the import + global boolean should do the trick