Add more runnable documentation (#13083)

- Adding documentation to the runnable.
- Documentation is not organized in the best way for the runnable; i.e.,
in
terms of LCEL vs. other standard methods, will follow up with more
edits.

libs/langchain/langchain/schema/runnable/base.py

@@ -82,18 +82,22 @@ Other = TypeVar("Other")
 class Runnable(Generic[Input, Output], ABC):
     """A unit of work that can be invoked, batched, streamed, transformed and composed.
 
-    Key methods:
+     Key Methods
+     ===========
 
     * invoke/ainvoke: Transforms a single input into an output.
     * batch/abatch: Efficiently transforms multiple inputs into outputs.
     * stream/astream: Streams output from a single input as it's produced.
     * astream_log: Streams output and selected intermediate results from an input.
 
-    Batch: By default, batch runs invoke() in parallel using a thread pool executor.
-           Override to optimize batching.
+    Built-in optimizations:
 
-    Async: Methods with "a" suffix are asynchronous. By default, they execute
-           the sync counterpart using asyncio's thread pool. Override for native async.
+    * Batch: By default, batch runs invoke() in parallel using a thread pool executor.
+             Override to optimize batching.
+
+    * Async: Methods with "a" suffix are asynchronous. By default, they execute
+             the sync counterpart using asyncio's thread pool.
+             Override for native async.
 
     All methods accept an optional config argument, which can be used to configure
     execution, add tags and metadata for tracing and debugging etc.
@@ -101,6 +105,9 @@ class Runnable(Generic[Input, Output], ABC):
     Runnables expose schematic information about their input, output and config via
     the input_schema property, the output_schema property and config_schema method.
 
+    LCEL and Composition
+    ====================
+
     The LangChain Expression Language (LCEL) is a declarative way to compose Runnables
     into chains. Any chain constructed this way will automatically have sync, async,
     batch, and streaming support.
@@ -115,6 +122,7 @@ class Runnable(Generic[Input, Output], ABC):
     to each. Construct it using a dict literal within a sequence or by passing a
     dict to RunnableParallel.
 
+
     For example,
 
     .. code-block:: python
@@ -133,6 +141,72 @@ class Runnable(Generic[Input, Output], ABC):
             'mul_5': RunnableLambda(lambda x: x * 5)
         }
         sequence.invoke(1) # {'mul_2': 4, 'mul_5': 10}
+
+    Standard Methods
+    ================
+
+    All Runnables expose additional methods that can be used to modify their behavior
+    (e.g., add a retry policy, add lifecycle listeners, make them configurable, etc.).
+
+    These methods will work on any Runnable, including Runnable chains constructed
+    by composing other Runnables. See the individual methods for details.
+
+    For example,
+
+    .. code-block:: python
+
+        from langchain.schema.runnable import RunnableLambda
+
+        import random
+
+        def add_one(x: int) -> int:
+            return x + 1
+
+
+        def buggy_double(y: int) -> int:
+            '''Buggy code that will fail 70% of the time'''
+            if random.random() > 0.3:
+                print('This code failed, and will probably be retried!')
+                raise ValueError('Triggered buggy code')
+            return y * 2
+
+        sequence = (
+            RunnableLambda(add_one) |
+            RunnableLambda(buggy_double).with_retry( # Retry on failure
+                stop_after_attempt=10,
+                wait_exponential_jitter=False
+            )
+        )
+
+        print(sequence.input_schema.schema()) # Show inferred input schema
+        print(sequence.output_schema.schema()) # Show inferred output schema
+        print(sequence.invoke(2)) # invoke the sequence (note the retry above!!)
+
+    Debugging and tracing
+    =====================
+
+    As the chains get longer, it can be useful to be able to see intermediate results
+    to debug and trace the chain.
+
+    You can set the global debug flag to True to enable debug output for all chains:
+
+        .. code-block:: python
+
+            from langchain.globals import set_debug
+            set_debug(True)
+
+    Alternatively, you can pass existing or custom callbacks to any given chain:
+
+       ... code-block:: python
+
+            from langchain.callbacks.tracers import ConsoleCallbackHandler
+
+            chain.invoke(
+                ...,
+                config={'callbacks': [ConsoleCallbackHandler()]}
+            )
+
+    For a UI (and much more) checkout LangSmith: https://docs.smith.langchain.com/
     """
 
     @property
@@ -169,7 +243,20 @@ class Runnable(Generic[Input, Output], ABC):
     def get_input_schema(
         self, config: Optional[RunnableConfig] = None
     ) -> Type[BaseModel]:
-        """The type of input this runnable accepts specified as a pydantic model."""
+        """Get a pydantic model that can be used to validate input to the runnable.
+
+        Runnables that leverage the configurable_fields and configurable_alternatives
+        methods will have a dynamic input schema that depends on which
+        configuration the runnable is invoked with.
+
+        This method allows to get an input schema for a specific configuration.
+
+        Args:
+            config: A config to use when generating the schema.
+
+        Returns:
+            A pydantic model that can be used to validate input.
+        """
         root_type = self.InputType
 
         if inspect.isclass(root_type) and issubclass(root_type, BaseModel):
@@ -187,7 +274,20 @@ class Runnable(Generic[Input, Output], ABC):
     def get_output_schema(
         self, config: Optional[RunnableConfig] = None
     ) -> Type[BaseModel]:
-        """The type of output this runnable produces specified as a pydantic model."""
+        """Get a pydantic model that can be used to validate output to the runnable.
+
+        Runnables that leverage the configurable_fields and configurable_alternatives
+        methods will have a dynamic output schema that depends on which
+        configuration the runnable is invoked with.
+
+        This method allows to get an output schema for a specific configuration.
+
+        Args:
+            config: A config to use when generating the schema.
+
+        Returns:
+            A pydantic model that can be used to validate output.
+        """
         root_type = self.OutputType
 
         if inspect.isclass(root_type) and issubclass(root_type, BaseModel):
@@ -278,14 +378,28 @@ class Runnable(Generic[Input, Output], ABC):
 
     @abstractmethod
     def invoke(self, input: Input, config: Optional[RunnableConfig] = None) -> Output:
-        """Transform a single input into an output. Override to implement."""
-        ...
+        """Transform a single input into an output. Override to implement.
+
+        Args:
+            input: The input to the runnable.
+            config: A config to use when invoking the runnable.
+               The config supports standard keys like 'tags', 'metadata' for tracing
+               purposes, 'max_concurrency' for controlling how much work to do
+               in parallel, and other keys. Please refer to the RunnableConfig
+               for more details.
+
+        Returns:
+            The output of the runnable.
+        """
 
     async def ainvoke(
         self, input: Input, config: Optional[RunnableConfig] = None, **kwargs: Any
     ) -> Output:
-        """
-        Default implementation of ainvoke, which calls invoke in a thread pool.
+        """Default implementation of ainvoke, calls invoke from a thread.
+
+        The default implementation allows usage of async code even if
+        the runnable did not implement a native async version of invoke.
+
         Subclasses should override this method if they can run asynchronously.
         """
         return await asyncio.get_running_loop().run_in_executor(
@@ -300,9 +414,12 @@ class Runnable(Generic[Input, Output], ABC):
         return_exceptions: bool = False,
         **kwargs: Optional[Any],
     ) -> List[Output]:
-        """
-        Default implementation of batch, which calls invoke N times.
-        Subclasses should override this method if they can batch more efficiently.
+        """Default implementation runs invoke in parallel using a thread pool executor.
+
+        The default implementation of batch works well for IO bound runnables.
+
+        Subclasses should override this method if they can batch more efficiently;
+        e.g., if the underlying runnable uses an API which supports a batch mode.
         """
         if not inputs:
             return []
@@ -333,9 +450,12 @@ class Runnable(Generic[Input, Output], ABC):
         return_exceptions: bool = False,
         **kwargs: Optional[Any],
     ) -> List[Output]:
-        """
-        Default implementation of abatch, which calls ainvoke N times.
-        Subclasses should override this method if they can batch more efficiently.
+        """Default implementation runs ainvoke in parallel using asyncio.gather.
+
+        The default implementation of batch works well for IO bound runnables.
+
+        Subclasses should override this method if they can batch more efficiently;
+        e.g., if the underlying runnable uses an API which supports a batch mode.
         """
         if not inputs:
             return []
@@ -682,6 +802,16 @@ class Runnable(Generic[Input, Output], ABC):
         *,
         exceptions_to_handle: Tuple[Type[BaseException], ...] = (Exception,),
     ) -> RunnableWithFallbacksT[Input, Output]:
+        """Add fallbacks to a runnable, returning a new Runnable.
+
+        Args:
+            fallbacks: A sequence of runnables to try if the original runnable fails.
+            exceptions_to_handle: A tuple of exception types to handle.
+
+        Returns:
+            A new Runnable that will try the original runnable, and then each
+            fallback in order, upon failures.
+        """
         from langchain.schema.runnable.fallbacks import RunnableWithFallbacks
 
         return RunnableWithFallbacks(