Add reference documentation for torch/library.h (pytorch#41470) (pytorch#41602)

Summary:
Pull Request resolved: pytorch#41470

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

Test Plan: Imported from OSS

Reviewed By: zou3519

Differential Revision: D22577426

Pulled By: ezyang

fbshipit-source-id: 4bfe5806061e74181a74d161c868acb7c1ecd1e4

Author: ezyang

docs/cpp/source/library.rst

@@ -0,0 +1,37 @@
+Torch Library API
+=================
+
+The PyTorch C++ API provides capabilities for extending PyTorch's core library
+of operators with user defined operators and data types.  Extensions implemented
+using the Torch Library API are made available for use in both the PyTorch eager
+API as well as in TorchScript.
+
+For a tutorial style introduction to the library API, check out the
+`Extending TorchScript with Custom C++ Operators
+<https://pytorch.org/tutorials/advanced/torch_script_custom_ops.html>`_
+tutorial.
+
+Macros
+------
+
+.. doxygendefine:: TORCH_LIBRARY
+
+.. doxygendefine:: TORCH_LIBRARY_IMPL
+
+Classes
+-------
+
+.. doxygenclass:: torch::Library
+  :members:
+
+.. doxygenclass:: torch::CppFunction
+  :members:
+
+Functions
+---------
+
+.. doxygengroup:: torch-dispatch-overloads
+  :content-only:
+
+.. doxygengroup:: torch-schema-overloads
+  :content-only: