Nevermind, some experimenting got me threads 1 (the one above), 5 and 6:
Thread 5
(gdb) bt
#0  0x00007ffff7a81f7a in pthread_cond_timedwait@@GLIBC_2.3.2 () from /usr/lib/libpthread.so.0
#1  0x00007ffff7b9c3ce in PyEval_RestoreThread () from /usr/lib/libpython3.7m.so.1.0
#2  0x00007ffff7b14c3e in ?? () from /usr/lib/libpython3.7m.so.1.0
#3  0x00007fffea180015 in torch::autograd::python::PythonEngine::thread_init(int) () from /home/mtgd/cpp-test/env/lib/python3.7/site-packages/torch/lib/libtorch_python.so
#4  0x00007fffeaaf75ef in execute_native_thread_routine () from /home/mtgd/cpp-test/env/lib/python3.7/site-packages/torch/_C.cpython-37m-x86_64-linux-gnu.so
#5  0x00007ffff7a7b4cf in start_thread () from /usr/lib/libpthread.so.0
#6  0x00007ffff7ed52d3 in clone () from /usr/lib/libc.so.6

Thread 6
(gdb) bt
#0  0x00007ffff7a81f7a in pthread_cond_timedwait@@GLIBC_2.3.2 () from /usr/lib/libpthread.so.0
#1  0x00007ffff7b9c3ce in PyEval_RestoreThread () from /usr/lib/libpython3.7m.so.1.0
#2  0x00007ffff7b14c3e in ?? () from /usr/lib/libpython3.7m.so.1.0
#3  0x00007fffea180015 in torch::autograd::python::PythonEngine::thread_init(int) () from /home/mtgd/cpp-test/env/lib/python3.7/site-packages/torch/lib/libtorch_python.so
#4  0x00007fffeaaf75ef in execute_native_thread_routine () from /home/mtgd/cpp-test/env/lib/python3.7/site-packages/torch/_C.cpython-37m-x86_64-linux-gnu.so
#5  0x00007ffff7a7b4cf in start_thread () from /usr/lib/libpthread.so.0
#6  0x00007ffff7ed52d3 in clone () from /usr/lib/libc.so.6

According to thread find . those are all.