make transform optional for DirectPosterior.

[janfb]

fixes #713

Before this PR, the DirectPosterior was constructing a theta_transform from the prior support by default and used it for maybe calculating the MAP. This can cause issues if a custom prior has a funky support.

This PR adds the theta_transform = None argument to the DirectPosterior top level so that users can pass an IdentityTransform to avoid the default construction of a transform to unconstrained space. If the argument is left None the support of the prior is used to construct a transform -- as before.

[codecov-commenter]

Codecov Report

Merging #714 (bf5bdaf) into main (d72fc6d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #714   +/-   ##
=======================================
  Coverage   73.85%   73.85%           
=======================================
  Files          79       79           
  Lines        5970     5971    +1     
=======================================
+ Hits         4409     4410    +1     
  Misses       1561     1561           

Flag	Coverage Δ
unittests	73.85% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
sbi/inference/posteriors/base_posterior.py	80.55% <ø> (-0.27%)	⬇️
sbi/inference/posteriors/direct_posterior.py	98.21% <100.00%> (ø)
.../inference/potentials/posterior_based_potential.py	100.00% <100.00%> (ø)

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