Remove gate_activation to allow pickling

[michaeldeistler]

Closes #1690

The lambda in the gate_activation leads to a pickling error. We could work around this, but I think it is really a niche feature and I would simply remove the option to modify the gate_activation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.62%. Comparing base (f2fd1ab) to head (3217e5e).
⚠️ Report is 4 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1694      +/-   ##
==========================================
- Coverage   88.48%   84.62%   -3.86%     
==========================================
  Files         137      137              
  Lines       11473    11470       -3     
==========================================
- Hits        10152     9707     -445     
- Misses       1321     1763     +442     

Flag	Coverage Δ
unittests	84.62% <100.00%> (-3.86%)	⬇️

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

Files with missing lines	Coverage Δ
sbi/neural_nets/net_builders/vector_field_nets.py	92.54% <100.00%> (-0.69%)	⬇️

... and 27 files with indirect coverage changes

[manuelgloeckler]

Looks good, fine with that.

A slightly more general approach would be to replace the lambda with an function i.e. def default_gate_activation on a global namespace and then default to this. This should be pickable.