FMPE with higher-dim conditions

[gmoss13]

Fixes #1702

This PR addresses the bug for FMPE posterior sampling when the condition event dim is greater than 1. I also added a test which checks that the samples are the correct shape, similar to posterior_nn_test.py::test_batched_score_sample_with_different_x, so there is some code repetition, but I think our minimal setup looks quite different when we are dealing with matrix variables so I think this is clearer than trying to fit it in the same test function.

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :warning: Please upload report for BASE (main@2c216c2). Learn more about missing BASE report. :white_check_mark: All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1704   +/-   ##
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  Coverage        ?   84.65%           
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  Files           ?      137           
  Lines           ?    11489           
  Branches        ?        0           
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  Hits            ?     9726           
  Misses          ?     1763           
  Partials        ?        0           

Flag	Coverage Δ
unittests	84.65% <100.00%> (?)

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Files with missing lines	Coverage Δ
sbi/inference/potentials/vector_field_potential.py	74.56% <100.00%> (ø)
sbi/samplers/ode_solvers/zuko_ode.py	100.00% <100.00%> (ø)