[Add] Dropout Operator

Context

Update the cpu.hpp file and add the following new files for the dropout implementation based on the Bernoulli distribution:

• DropoutImp_forward_kernels.hpp
• DropoutImp.hpp
• DropoutImpl.cpp

Part of: aidge_core#183

Modified files

• aidge/aidge_backend_cpu/include/aidge/backend/cpu.hpp

Added files

• aidge_backend_cpu/include/aidge/backend/cpu/operator/DropoutImpl_forward_kernels.hpp
• aidge_backend_cpu/include/aidge/backend/cpu/operator/DropoutImpl.hpp
• aidge_backend_cpu/src/operator/DropoutImpl.cpp
• add: unit-test Test_DropoutImpl.cpp