Thank you. This one ran without issues:
xcode-select --install
Then I tried again from scratch the:
MACOSX_DEPLOYMENT_TARGET=10.9 CC=clang CXX=clang++ TORCH_CUDA_ARCH_LIST=6.1 python3 setup.py install
Which got me further, but as some others in the thread you point it did not solve all.
Now it breaks at 75%:
make[1]: *** [caffe2/CMakeFiles/torch.dir/all] Error 2
make: *** [all] Error 2
Traceback (most recent call last):
File “setup.py”, line 759, in 
build_deps()
File “setup.py”, line 321, in build_deps
cmake=cmake)
I cannot see by myself where the problem could come from (but it seems pytorch related) … may you have any more guesses please ?

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