[pybind] Add pybind API reference docs

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Author: larryliu0820

docs/source/runtime-pybind-api-reference.rst

@@ -0,0 +1,18 @@
+Runtime Pybind API Reference
+----------------------------------
+Pybind APIs can load a serialized .pte file (see `Export to ExecuTorch Tutorial <tutorials/export-to-executorch-tutorial.html>`__ for how to get .pte file from a PyTorch `nn.Module`) and execute it using `torch.Tensor` as inputs, in a Python environment. The result also comes back as `torch.Tensor` so it can be used as a quick way to validate the correctness of the program.
+
+For detailed information on how APIs evolve and the deprecation process, please refer to the `ExecuTorch API Life Cycle and Deprecation Policy <api-life-cycle.html>`__.
+
+.. automodule:: executorch.runtime
+.. autoclass:: Runtime
+    :members: get, load_program
+
+.. autoclass:: OperatorRegistry
+    :members: operator_names
+
+.. autoclass:: Program
+    :members: method_names, load_method
+
+.. autoclass:: Method
+    :members: execute, metadata

runtime/__init__.py

@@ -107,6 +107,9 @@ def __init__(self, module: ExecuTorchModule, data: Optional[bytes]) -> None:
 
     @property
     def method_names(self) -> Set[str]:
+        """
+        Returns method names of the `Program` as a set of strings.
+        """
         return set(self._methods.keys())
 
     def load_method(self, name: str) -> Optional[Method]:
@@ -130,7 +133,9 @@ def __init__(self, legacy_module: ModuleType) -> None:
 
     @property
     def operator_names(self) -> Set[str]:
-        """The names of all registered operators."""
+        """
+        Returns the names of all registered operators as a set of strings.
+        """
         return set(self._legacy_module._get_operator_names())