add more docs

Signed-off-by: Jeffrey Kinard <[email protected]>

sdks/python/apache_beam/yaml/yaml_ml.py

@@ -86,9 +86,12 @@ def underlying_handler(self):
   @staticmethod
   def default_preprocess_fn():
     raise ValueError(
-        'Handler does not implement a default preprocess '
+        'Model Handler does not implement a default preprocess '
         'method. Please define a preprocessing method using the '
-        '\'preprocess\' tag.')
+        '\'preprocess\' tag. This is required in most cases because '
+        'most models will have a different input shape, so the model '
+        'cannot generalize how the input Row should be transformed. For '
+        'an example preprocess method, see VertexAIModelHandlerJSONProvider')
 
   def _preprocess_fn_internal(self):
     return lambda row: (row, self._preprocess_fn(row))
@@ -134,17 +137,34 @@ def __init__(
       project: str,
       location: str,
       preprocess: Dict[str, str],
+      postprocess: Optional[Dict[str, str]] = None,
       experiment: Optional[str] = None,
       network: Optional[str] = None,
       private: bool = False,
       min_batch_size: Optional[int] = None,
       max_batch_size: Optional[int] = None,
       max_batch_duration_secs: Optional[int] = None,
-      env_vars: Optional[Dict[str, Any]] = None,
-      postprocess: Optional[Dict[str, str]] = None):
+      env_vars: Optional[Dict[str, Any]] = None):
     """
     ModelHandler for Vertex AI.
 
+    This Model Handler can be used with RunInference to load a model hosted
+    on VertexAI. Every model that is hosted on VertexAI should have three
+    distinct, required, parameters - `endpoint_id`, `project` and `location`.
+    These parameters tell the Model Handler how to access the model's endpoint
+    so that input data can be sent using an API request, and inferences can be
+    received as a response.
+
+    This Model Handler also required a `preprocess` function to be defined.
+    Preprocessing and Postprocessing are described in more detail in the
+    RunInference docs:
+      https://beam.apache.org/releases/yamldoc/current/#runinference
+
+    Every model will have a unique input, but all requests should be
+    JSON-formatted. For example, most language models such as Llama and Gemma
+    expect a JSON with the key "prompt" (among other optional keys). In Python,
+    JSON can be expressed as a dictionary.
+
     For example: ::
 
         - type: RunInference
@@ -159,10 +179,24 @@ def __init__(
                 preprocess:
                   callable: 'lambda x: {"prompt": x.prompt, "max_tokens": 50}'
 
+    In the above example, which mimics a call to a Llama 3 model hosted on
+    VertexAI, the preprocess function (in this case a lambda) takes in a Beam
+    Row with a single field, "prompt", and maps it to a dict with the same
+    field. It also specifies an optional parameter, "max_tokens", that tells the
+    model the allowed token size (in this case input + output token size).
+
     Args:
       endpoint_id: the numerical ID of the Vertex AI endpoint to query.
       project: the GCP project name where the endpoint is deployed.
       location: the GCP location where the endpoint is deployed.
+      preprocess: A python callable, defined either inline, or using a file,
+        that is invoked on the input row before sending to the model to be
+        loaded by this ModelHandler. This parameter is required by the
+        `VertexAIModelHandlerJSON` ModelHandler.
+      postprocess: A python callable, defined either inline, or using a file,
+        that is invoked on the PredictionResult output by the ModelHandler
+        before parsing into the output Beam Row under the field name defined
+        by the inference_tag.
       experiment: Experiment label to apply to the
         queries. See
         https://cloud.google.com/vertex-ai/docs/experiments/intro-vertex-ai-experiments
@@ -183,14 +217,6 @@ def __init__(
       max_batch_duration_secs: The maximum amount of time to buffer
         a batch before emitting; used in streaming contexts.
       env_vars: Environment variables.
-      preprocess: A python callable, defined either inline, or using a file,
-        that is invoked on the input row before sending to the model to be
-        loaded by this ModelHandler. This parameter is required by the
-        `VertexAIModelHandlerJSON` ModelHandler.
-      postprocess: A python callable, defined either inline, or using a file,
-        that is invoked on the PredictionResult output by the ModelHandler
-        before parsing into the output Beam Row under the field name defined
-        by the inference_tag.
     """
 
     try:
@@ -222,10 +248,6 @@ def inference_output_type(self):
     return RowTypeConstraint.from_fields([('example', Any), ('inference', Any),
                                           ('model_id', Optional[str])])
 
-  @staticmethod
-  def default_postprocess_fn():
-    return lambda x: beam.Row(**x._asdict())
-
 
 @beam.ptransform.ptransform_fn
 def run_inference(