Add docs for how to mark as unbacked (#154822)

bobrenjc93 · pytorchmergebot · commit d99e9568ec8c · 2025-06-02T18:30:57.000Z
Pull Request resolved: #154822 Approved by: https://github.com/Skylion007 ghstack dependencies: #154802, #154826
diff --git a/docs/source/torch.compiler_dynamic_shapes.rst b/docs/source/torch.compiler_dynamic_shapes.rst
@@ -128,6 +128,8 @@ Naively implemented, this is too restrictive: most PyTorch programs will immedia
 - Even if nontrivial compute is needed, sometimes a property is never actually queried at all. Making these precomputed properties lazy allows us to avoid guarding on an unbacked symbolic integer unless it is actually needed.
 - The data in an integer tensor is generally not known to be non-negative. However, we provide an API ``constrain_range`` whereby a user can specify that a size is bounded above and below by known limits.
 
+Similar to the dynamic APIs, there are corresponding unbacked APIs: namely you can use mark_unbacked instead of ``mark_dynamic`` and ``TORCH_COMPILE_UNBACKED_SOURCES`` instead of ``TORCH_COMPILE_DYNAMIC_SOURCES`` to tell the compiler to mark an input as unbacked.
+
 In future versions of PT2 (beyond PT2.1), we will extend our reasoning system
 to infer that an unbacked symbolic integer is size-like based on usage.  For
 example, if you pass the result of an ``.item()`` call to a factory function
