[RF] Prepare RooFit and HistFactory for ROOT without implicit ownership

[hageboeck]

This takes a few RooFit-related commits out of #18083.

• The backend-related commit fixes the tests in case root has been configured with -Dclad=off.
• The other two move over to explicitly registering/writing histograms instead of relying on implicit ownership. This way, the tests are independent of whether implicit registration is used or not.

Note: This might need #20888 to be merged first, in order not to issue warnings about histograms being replaced.

[guitargeek]

See inline comment

[guitargeek]

This should not be necessary because RooFit::EvalBackend::CodegenNoGrad() is available also without Clad, and ROOFIT_EVAL_BACKENDS is already defined to be empty if ROOFIT_CLAD is not defined. The problem must be somewhere else I think

[hageboeck]

The failure is:
In gtest-roofit-histfactory-testHistFactory

 [#0] ERROR:InputArguments -- RooFit was built without clad. Codegen backends are unavailable. Falling back to default.
314: 
314: [ FATAL ] /usr/include/gtest/internal/gtest-param-util.h:589:: Condition test_param_names.count(param_name) == 0 failed. Duplicate parameterized test name 'EquidistantBins_OverallSyst_Backend_cpu', in /home/stephan/code/root/roofit/histfactory/test/testHistFactory.cxx line 706

And in gtest-roofit-roofitcore-testRooMinimizer:

 [#0] ERROR:InputArguments -- RooFit was built without clad. Codegen backends are unavailable. Falling back to default.
387: 
387: [ FATAL ] /usr/include/gtest/internal/gtest-param-util.h:589:: Condition test_param_names.count(param_name) == 0 failed. Duplicate parameterized test name 'EvalBackendcpu', in /home/stephan/code/root/roofit/roofitcore/test/testRooMinimizer.cxx line 163

Running the preprocessor on the latter, I see that ROOFIT_EVAL_BACKENDS_WITH_CODEGEN expands to
RooFit::EvalBackend::Legacy(), RooFit::EvalBackend::Cpu(), RooFit::EvalBackend::CodegenNoGrad()

The return values of these functions are (when clad is off):
Legacy, Cpu, Cpu. The doubling of Cpu is why gtest aborts the test.

I fixed this by not adding CodegenNoGrad(), so the macro would expand to
Legacy, Cpu

[guitargeek]

Ok, then the problem is that CodegenNoGrad() unnecessarily falls back to Cpu. I'll see how I can fix this.

[guitargeek]

PR opened:

• [RF] Don't fall back to default backend for codegen_no_grad if clad=OFF #20890