[tmva][sofie] throw on invalid BatchNormalization input count

[harz05]

Changes:

ParseBatchNormalization only constructed the operator when input_size() == 5 but had no else clause, so nodes with fewer inputs silently returned a null unique_ptr and crashed later in Generate() with an unrelated error message.

I've simply added an else throw to reject invalid input counts immediately at parse time with a clear error message, consistent with other parsers in the codebase.

Checklist:

• tested changes locally
• updated the docs (if necessary)

This PR fixes #21759

[harz05]

I'd also like to add a regression test for this(verifying Parse() throws std::runtime_error) but im not sure what's the best to add it.
TestCustomModelsFromONNX.cxx doesn't currently link against ROOTTMVASofieParser and placing an invalid .onnx in input_models/ would break the emitFromONNX fixture.
@guitargeek

[github-actions]

Test Results

    22 files      22 suites   3d 5h 19m 29s ⏱️
 3 836 tests  3 835 ✅  1 💤 0 ❌
75 716 runs  75 698 ✅ 18 💤 0 ❌

Results for commit 6141a69.

[guitargeek]

Thanks! I don't think we need a regression test here, as this doesn't fix a bug per se but is more a usability improvement 👍

[harz05]

Thanks! I don't think we need a regression test here, as this doesn't fix a bug per se but is more a usability improvement 👍

Thanks for the review and merge!