fix: nflows inverse transform and retarget lc2st

[janfb]

when working on the LC2ST tests I noticed a typo in the inverse_transform convenience function for LC2ST and then noticed that it is actually never used.

This PR fixes the typo and then replaces the lambda functions to npe._net.transform with the existing helper in the tutorials and the tests.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.90%. Comparing base (839762d) to head (adca39a).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1865      +/-   ##
==========================================
+ Coverage   87.76%   87.90%   +0.13%     
==========================================
  Files         143      143              
  Lines       13317    13402      +85     
==========================================
+ Hits        11688    11781      +93     
+ Misses       1629     1621       -8     

Flag	Coverage Δ
fast	81.39% <100.00%> (?)

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

Files with missing lines	Coverage Δ
sbi/neural_nets/estimators/nflows_flow.py	87.50% <100.00%> (+25.00%)	⬆️

... and 1 file with indirect coverage changes