Update on "[pytorch] Add triplet margin loss with custom distance"

Summary: As discussed [here](#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

Reviewers:

Subscribers:

Tasks:

Tags:

Differential Revision: [D23363898](https://our.internmc.facebook.com/intern/diff/D23363898)

[ghstack-poisoned]

torch/nn/functional.py

@@ -11,7 +11,7 @@
 from .modules.utils import _single, _pair, _triple, _list_with_default
 from . import grad  # noqa: F401
 from torch import _VF
-from .._jit_internal import boolean_dispatch, Callable, List, Optional, _overload
+from .._jit_internal import boolean_dispatch, List, Optional, _overload
 from ..overrides import has_torch_function, handle_torch_function