Use a single point to process non-differentiable functions in the reverse mode

[PetroZarytskyi]

Right now, we have a bunch of reasons to consider a function non-differentiable and not produce a pullback. In all cases, we clone the call and return early in different ways, almost all of which have flaws. For example:

Return on line 1488: it's not necessary to pass 0 as the adjoint of non-differentiable entities in the reverse mode. Also, cloning arguments is dangerous for multiple reasons (e.g. sometimes we introduce operators, brackets, etc. which are not generated when cloning) and doesn't account for their possible differentiable side effects.

Return on line 1520: Again, cloning is dangerous.

Return on line 1642: Cloning again. Also, providing the cloned call as the adjoint doesn't make much sense.

Return on line 1664: dfdx() of the call shouldn't be passed to its arguments.

This PR replaces all of these cases to the same place and fixes the mentioned issues.

[vgvassilev]

LGTM!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.55%. Comparing base (6228385) to head (30c72ae).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1161      +/-   ##
==========================================
- Coverage   94.55%   94.55%   -0.01%     
==========================================
  Files          51       51              
  Lines        8933     8923      -10     
==========================================
- Hits         8447     8437      -10     
  Misses        486      486              

Files with missing lines	Coverage Δ
lib/Differentiator/ReverseModeVisitor.cpp	95.56% <100.00%> (-0.02%)	⬇️

Files with missing lines	Coverage Δ
lib/Differentiator/ReverseModeVisitor.cpp	95.56% <100.00%> (-0.02%)	⬇️