Adapt usage of log_abs_det_jacobian for torch>=1.8

[janfb]

The dependence on torch 1.8 makes sense because the current sbi version, which we want to use, depends on it.

With this change we get rid of the helper function get_log_abs_det_jacobian that was distinguishing between the behavior before and after torch 1.8 and was doing the summation explicitly.

Details:

• with the dependence on torch>=1.8 the output of log_prob and log_abs_det_jacobian changes: when the input has several parameter dimensions the output will keep those dimensions and we would have to sum over them by hand to get the joint log_prob over parameter dimensions.
• this can be prevented by "reinterpreting" them as batch dimensions.
• for transforms this works via the IndependentTransform as a wrapper

See also #15

[janfb]

we could add the fixed dependency on sbi in this branch as well, once the new sbi version is released.

[jan-matthis]

Great, cheers!