fix: add batch dim to inv transform, add test.

[janfb]

closes #841

as suggested in #650 we need a batch dim when calling the inverse transform that was constructed from a MultipleIndependent prior (because of the CatTransform).

Thus, the fix is to ensure the batch dim in the optimization routines for map.

[codecov]

Codecov Report

Merging #867 (5e3bd38) into main (8769e25) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #867      +/-   ##
==========================================
+ Coverage   74.60%   74.65%   +0.04%     
==========================================
  Files          80       80              
  Lines        6242     6242              
==========================================
+ Hits         4657     4660       +3     
+ Misses       1585     1582       -3     

Flag	Coverage Δ
unittests	74.65% <ø> (+0.04%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
sbi/utils/sbiutils.py	80.22% <ø> (ø)

... and 2 files with indirect coverage changes

[michaeldeistler]

LGTM, thanks!