[RF] Don't disable dirty flag propagation in generation contexts

We should avoid to disable RooFits internal caching if not well
justified, because not doing the caching can lead to severe performance
hits like in #21788.

There are some places in RooFit where the caching is disabled on
purpose, because sometimes the overhead of the bookkeeping for caching
can occasionally be more expensive than model evalution itself. However,
this should not be done in general. One place where this happened were
the **RooGenContext** and **RooAddGenContext** helper classes for toy
dataset sampling via `RooAbsPdf::generate()`, and this commit suggests
to remove it. The performance hit that it incurres if the model is
actually expensive to evalutate is too high.

Partialy closes #21788.

Author: guitargeek

roofit/roofitcore/src/RooAddGenContext.cxx

@@ -58,7 +58,6 @@ RooAddGenContext::RooAddGenContext(const RooAddPdf &model, const RooArgSet &vars
   _pdfSet = std::make_unique<RooArgSet>();
   RooArgSet(model).snapshot(*_pdfSet, true);
   _pdf = static_cast<RooAddPdf*>(_pdfSet->find(model.GetName())) ;
-  _pdf->setOperMode(RooAbsArg::ADirty,true) ;
 
   // Fix normalization set of this RooAddPdf
   if (prototype)

roofit/roofitcore/src/RooGenContext.cxx

@@ -82,7 +82,6 @@ RooGenContext::RooGenContext(const RooAbsPdf &model, const RooArgSet &vars,
 
   // Find the clone in the snapshot list
   _pdfClone = static_cast<RooAbsPdf*>(_cloneSet.find(model.GetName()));
-  _pdfClone->setOperMode(RooAbsArg::ADirty,true) ;
 
   // Optionally fix RooAddPdf normalizations
   if (prototype&&_pdfClone->dependsOn(*prototype->get())) {
@@ -322,10 +321,6 @@ void RooGenContext::attach(const RooArgSet& args)
 
 void RooGenContext::initGenerator(const RooArgSet &theEvent)
 {
-  for (auto* arg : theEvent) {
-    arg->setOperMode(RooAbsArg::ADirty) ;
-  }
-
   attach(theEvent) ;
 
   // Reset the cloned model's error counters.