[NFC][MLGO] Use std::move when moving protobufs

Because of an odd linking problem, we need to temporarily support
building with TF C API 1.15 + tensorflow 2.50 pip package in
'development' mode scenarios. Protobuf Message 'Swap' is partially
implemented in the header (2.50) and relies on a symbol not found in TF
C API 1.15. std::move avoids that, at no semantic cost.