Clarify Python custom operators do not work without Python (#3212)

See pytorch/pytorch#143786

advanced_source/custom_ops_landing_page.rst

@@ -23,6 +23,7 @@ You may wish to author a custom operator from Python (as opposed to C++) if:
   respect to ``torch.compile`` and ``torch.export``.
 - you have some Python bindings to C++/CUDA kernels and want those to compose with PyTorch
   subsystems (like ``torch.compile`` or ``torch.autograd``)
+- you are using Python (and not a C++-only environment like AOTInductor).
 
 Integrating custom C++ and/or CUDA code with PyTorch
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

advanced_source/python_custom_ops.py

@@ -30,6 +30,12 @@
   into the function).
 - Adding training support to an arbitrary Python function
 
+Use :func:`torch.library.custom_op` to create Python custom operators.
+Use the C++ ``TORCH_LIBRARY`` APIs to create C++ custom operators (these
+work in Python-less environments).
+See the `Custom Operators Landing Page <https://pytorch.org/tutorials/advanced/custom_ops_landing_page.html>`_
+for more details.
+
 Please note that if your operation can be expressed as a composition of
 existing PyTorch operators, then there is usually no need to use the custom operator
 API -- everything (for example ``torch.compile``, training support) should