Add STHN method for heterogeneous temporal graphs

[erfanloghmani]

Adding STHN method from: https://github.com/celi52/STHN/tree/main

To run:

1. Install requirements. Also updated requirements.txt. The two new requirements are pybind11 and torchmetrics==0.11.0
2. compile the sampler

   python sthn_sampler_setup.py build_ext --inplace

3. run the example code

   python sthn.py

If the code runs correctly the output would end with

INFO: Test: Evaluation Setting: >>> ONE-VS-MANY <<< 
        Test: mrr: X
        Test: Elapsed Time (s):  Y

[shenyangHuang]

Thanks Erfan, can you also add a README in the code so we keep track of it after it is merged? Just copy paste the install instruction and anything else you want to add into a STHN_README.md