[pytorch] Add triplet margin loss with custom distance (#43680)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43680
As discussed [here](https://github.com/pytorch/pytorch/issues/43342),
adding in a Python-only implementation of the triplet-margin loss that takes a
custom distance function. Still discussing whether this is necessary to add to
PyTorch Core.
Test Plan:
python test/run_tests.py
Imported from OSS
Reviewed By: albanD
Differential Revision: D23363898
fbshipit-source-id: 1cafc05abecdbe7812b41deaa1e50ea11239d0cb
