Document generate HTTP endpoint (#6412)

* Document generate HTTP endpoint

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docs/protocol/extension_generate.md

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+
+# Generate Extension
+
+> [!NOTE]
+> The Generate Extension is *provisional* and likely to change in future versions.
+
+This document describes Triton's generate extension. The generate
+extension provides a simple text-oriented endpoint schema for interacting with
+large language models (LLMs). The generate endpoint is specific to HTTP/REST
+frontend.
+
+## HTTP/REST
+
+In all JSON schemas shown in this document, `$number`, `$string`, `$boolean`,
+`$object` and `$array` refer to the fundamental JSON types. #optional
+indicates an optional JSON field.
+
+Triton exposes the generate endpoint at the following URLs. The client may use
+HTTP POST request to different URLs for different response behavior, the
+endpoint will return the generate results on success or an error in the case of
+failure.
+
+```
+POST v2/models/${MODEL_NAME}[/versions/${MODEL_VERSION}]/generate
+
+POST v2/models/${MODEL_NAME}[/versions/${MODEL_VERSION}]/generate_stream
+```
+
+### generate v.s. generate_stream
+
+Both URLs expect the same request JSON object, and generate the same response
+JSON object. However, `generate` returns exactly 1 response JSON object, while
+`generate_stream` may return multiple responses based on the inference
+results. `generate_stream` returns the responses as
+[Server-Sent Events](https://html.spec.whatwg.org/multipage/server-sent-events.html#server-sent-events)
+(SSE), where each response will be a "data" chunk in the HTTP response body.
+Also, note that an error may be returned during inference, whereas the HTTP
+response code has been set in the first response of the SSE, which can result in
+receiving an [error object](#generate-response-json-error-object) while status
+code shows success (200). Therefore the user must always check whether an error
+object is received when generating responses through `generate_stream`.
+
+### Generate Request JSON Object
+
+The generate request object, identified as *$generate_request*, is
+required in the HTTP body of the POST request. The model name and
+(optionally) version must be available in the URL. If a version is not
+provided, the server may choose a version based on its own policies or
+return an error.
+
+    $generate_request =
+    {
+      "text_input" : $string,
+      "parameters" : $parameters #optional
+    }
+
+* "text_input" : The text input that the model should generate output from.
+* "parameters" : An optional object containing zero or more parameters for this
+  generate request expressed as key/value pairs. See
+  [Parameters](#parameters) for more information.
+
+> [!NOTE]
+> Any additional properties in the request object are passed either as
+> parameters or tensors based on model specification.
+
+#### Parameters
+
+The *$parameters* JSON describes zero or more “name”/”value” pairs,
+where the “name” is the name of the parameter and the “value” is a
+$string, $number, or $boolean.
+
+    $parameters =
+    {
+      $parameter, ...
+    }
+
+    $parameter = $string : $string | $number | $boolean
+
+Parameters are model-specific. The user should check with the model
+specification to set the parameters.
+
+#### Example Request
+
+Below is an example to send generate request with additional model parameters `stream` and `temperature`.
+
+```
+$ curl -X POST localhost:8000/v2/models/mymodel/generate -d '{"text_input": "client input", "parameters": {"stream": false, "temperature": 0}}'
+
+POST /v2/models/mymodel/generate HTTP/1.1
+Host: localhost:8000
+Content-Type: application/json
+Content-Length: <xx>
+{
+  "text_input":  "client input",
+  "parameters" :
+    {
+      "stream": false,
+      "temperature": 0
+    }
+}
+```
+
+### Generate Response JSON Object
+
+A successful generate request is indicated by a 200 HTTP status code.
+The generate response object, identified as *$generate_response*, is returned in
+the HTTP body.
+
+    $generate_response =
+    {
+      "model_name" : $string,
+      "model_version" : $string,
+      "text_output" : $string
+    }
+
+* "model_name" : The name of the model used for inference.
+* "model_version" : The specific model version used for inference.
+* "text_output" : The output of the inference.
+
+#### Example Response
+
+```
+200
+{
+  "model_name" : "mymodel",
+  "model_version" : "1",
+  "text_output" : "model output"
+}
+```
+
+### Generate Response JSON Error Object
+
+A failed generate request must be indicated by an HTTP error status
+(typically 400). The HTTP body must contain the
+*$generate_error_response* object.
+
+    $generate_error_response =
+    {
+      "error": <error message string>
+    }
+
+* “error” : The descriptive message for the error.
+
+#### Example Error
+
+```
+400
+{
+  "error" : "error message"
+}
+```