Fix compiler warnings caused by implicit type conversion (fixes #3677)

[btrotta]

Fix compiler warnings C4244 and C4267

[StrikerRUS]

Great! Only few lines are left:

2021-01-05T08:26:16.1022300Z   serial_tree_learner.cpp
2021-01-05T08:26:29.1103930Z   tree_learner.cpp
2021-01-05T08:26:29.1115764Z C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\VC\Tools\MSVC\14.16.27023\include\xutility(2903): warning C4244: '=': conversion from 'const _Ty' to 'float', possible loss of data [D:\a\1\s\build\_lightgbm.vcxproj]
2021-01-05T08:26:29.3073256Z           with
2021-01-05T08:26:29.3912834Z           [
2021-01-05T08:26:29.5125855Z               _Ty=int
2021-01-05T08:26:30.0013145Z           ] (compiling source file D:\a\1\s\src\treelearner\linear_tree_learner.cpp)
2021-01-05T08:26:30.3244783Z   C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\VC\Tools\MSVC\14.16.27023\include\xutility(2917): note: see reference to function template instantiation 'void std::_Fill_unchecked1<_FwdIt,_Ty>(_FwdIt,_FwdIt,const _Ty &,std::false_type)' being compiled
2021-01-05T08:26:30.7134387Z           with
2021-01-05T08:26:31.1602801Z           [
2021-01-05T08:26:31.5463919Z               _FwdIt=float *,
2021-01-05T08:26:32.0704575Z               _Ty=int
2021-01-05T08:26:32.4605207Z           ] (compiling source file D:\a\1\s\src\treelearner\linear_tree_learner.cpp)
2021-01-05T08:26:32.8519676Z   voting_parallel_tree_learner.cpp
2021-01-05T08:26:33.2523229Z   C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\VC\Tools\MSVC\14.16.27023\include\xutility(2925): note: see reference to function template instantiation 'void std::_Fill_unchecked<float*,_Ty>(_FwdIt,_FwdIt,const _Ty &)' being compiled
2021-01-05T08:26:33.4811651Z           with
2021-01-05T08:26:33.8333686Z           [
2021-01-05T08:26:34.2263103Z               _Ty=int,
2021-01-05T08:26:34.6213071Z               _FwdIt=float *
2021-01-05T08:26:35.0073728Z           ] (compiling source file D:\a\1\s\src\treelearner\linear_tree_learner.cpp)
2021-01-05T08:26:35.3980723Z   D:\a\1\s\src\treelearner\linear_tree_learner.cpp(237): note: see reference to function template instantiation 'void std::fill<std::_Vector_iterator<std::_Vector_val<std::_Simple_types<_Ty>>>,int>(_FwdIt,_FwdIt,const int &)' being compiled
2021-01-05T08:26:35.7825747Z           with
2021-01-05T08:26:36.1753974Z           [
2021-01-05T08:26:36.5593124Z               _Ty=LightGBM::label_t,
2021-01-05T08:26:36.8104822Z               _FwdIt=std::_Vector_iterator<std::_Vector_val<std::_Simple_types<LightGBM::label_t>>>
2021-01-05T08:26:37.1303753Z           ]
2021-01-05T08:26:37.1331979Z   D:\a\1\s\src\treelearner\linear_tree_learner.cpp(124): note: see reference to function template instantiation 'void LightGBM::LinearTreeLearner::CalculateLinear<true>(LightGBM::Tree *,bool,const LightGBM::score_t *,const LightGBM::score_t *,bool) const' being compiled
2021-01-05T08:26:42.3756298Z   c_api.cpp

[btrotta]

@StrikerRUS Sorry I missed those, fixed now

[StrikerRUS]

Awesome! I can confirm there are no warnings at Windows machine any more. Thanks a lot!

[github-actions]

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