Allow undefined libraries to fix CMake errors

Summary:
This should get rid of the `libtorch_cpu.so: undefined reference to `logf@GLIBC_2.27'` etc.. we see in our cpp unit tests on Github CI See: https://fb.workplace.com/groups/1405155842844877/permalink/23944202491846891/

Note there is another error of undefined reference - but it's related to our library not installing google benchmark rather than something inside of torch.so file like the ones mentioned in post.

Differential Revision: D71862824

Author: aporialiao

.github/workflows/cpp_unittest_ci_cpu.yml

@@ -1,7 +1,7 @@
 # This workflow will install Python dependencies, run tests and lint with a variety of Python versions
 # For more information see: https://help.github.com/actions/language-and-framework-guides/using-python-with-github-actions
 
-name: CPU Unit Test C++ CI
+name: C++ CPU Unit Test CI
 
 on:
   push:
@@ -58,6 +58,7 @@ jobs:
         conda run -n build_binary cmake \
             -DBUILD_TEST=ON \
             -DBUILD_REDIS_IO=ON \
+            -DCMAKE_EXE_LINKER_FLAGS=-Wl,--allow-shlib-undefined \
             -DCMAKE_PREFIX_PATH=/opt/conda/envs/build_binary/lib/python${{ matrix.python-version }}/site-packages/torch/share/cmake ..
         conda run -n build_binary make -j
         conda run -n build_binary ctest -V .