Most likely you’re not using pytorch you installed but a system-wide installed one. If you don’t have sudo access you won’t have been able to install torch into system-wide dirs, and your error message shows that it a system-wide torch you’re trying to use.
Before you edited this last comment - you shared how you installed it - so based on that info we can validate that the distributed package is whole:
$ wget https://download.pytorch.org/whl/cu111/torch-1.8.1%2Bcu111-cp38-cp38-linux_x86_64.whl
$ unzip torch-1.8.1+cu111-cp38-cp38-linux_x86_64.whl
$ find torch | grep pybind11.h
torch/include/pybind11/pybind11.h

The header is there.
Whoever installed torch on that system, didn’t do a full install. And you’re attempting to use the system-wide torch and not the torch you presumably installed yourself.
I recommend you create a dedicated conda env (or any other preferred python virt env of your choice), activate it, install torch and whatever else you need and everything will just work.
Of course, make sure you’re inside the virtual environment when you run your code.
Alternatively contact your sysadmin and request installing the missing files. I was guessing that this was some kind of custom .deb package which only installed the essentials and left out any dev files, hence the discussion of apt.
Of course, you can also do pip install pybind11 to solve the immediate problem, but chances are that some other required by torch header files will be missing. So it’s best to solve this problem at the root.