[Functionalization] Fix ScatterReduce (#4576)
Summary:
ScatterReduce::reduce_ uses a unsafe type c10::string_view to store the
string. Replace it with std::string and re-enable all previous failing tests.
Test Plan:
TPU_LIBRARY_PATH=/home/ptxla/.local/lib/python3.8/site-packages/libtpu/libtpu.so PJRT_DEVICE=CPU test/cpp/build/test_ptxla --gtest_filter=AtenXlaTensorTest.TestScatterReduce*
