Fix calculation of number of bins in FindGroup

[CVPaul]

Fixes #3339.

The calculation method for num_total_bin used during Exclusive Feature Bundling

LightGBM/src/io/dataset.cpp

Line 134 in 665c473

(bin_mappers[fidx]->GetDefaultBin() == 0 ? -1 : 0);
doesn't align completely with the way num_total_bin is calculated during the creation of a FeatureGroup
LightGBM/include/LightGBM/feature_group.h

Line 68 in 665c473

if (bin_mappers_[i]->GetMostFreqBin() == 0) {
As a result, the max_bin_per_group (=256) is working during Bundling, but it is not working when creating the FeatureGroup. When I replaced the GetDefaultBin() at dataset.cpp#L134 with GetMostFreqBin(), the issue was resolved.

[CVPaul]

@microsoft-github-policy-service agree

[jameslamb]

@shiyu1994 could you please edit the title of this to make it more informative in the release notes? I would, but I don't really understand the effect of this PR.

[Borda]

this would be great to have in any release soon, please 🙏

[fillipemadureira]

this would be great to have in any release soon, please 🙏

I've just come across this issue. No release with this solution yet??

[karta282950]

I also meet same problem. When release this version?

[borchero]

Unfortunately, we are currently battling CI issues (cf. #6425). Once we got our GPU runners working again, we are planning to prepare another release -- this will likely happen in the early part of May.

[github-actions]

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