Add support for multi part chat messages (and gpt-4-vision-preview model) (#580)

* Add support for multi part chat messages

OpenAI has recently introduced a new model called gpt-4-visual-preview,
which now supports images as input. The chat completion endpoint accepts
multi-part chat messages, where the content can be an array of structs
in addition to the usual string format.

This commit introduces new structures and constants to represent
different types of content parts. It also implements the json.Marshaler
and json.Unmarshaler interfaces on ChatCompletionMessage.

* Add ImageURLDetail and ChatMessagePartType types

* Optimize ChatCompletionMessage deserialization

* Add ErrContentFieldsMisused error

chat_test.go

@@ -3,6 +3,7 @@ package openai_test
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"net/http"
@@ -296,6 +297,108 @@ func TestAzureChatCompletions(t *testing.T) {
 	checks.NoError(t, err, "CreateAzureChatCompletion error")
 }
 
+func TestMultipartChatCompletions(t *testing.T) {
+	client, server, teardown := setupAzureTestServer()
+	defer teardown()
+	server.RegisterHandler("/openai/deployments/*", handleChatCompletionEndpoint)
+
+	_, err := client.CreateChatCompletion(context.Background(), openai.ChatCompletionRequest{
+		MaxTokens: 5,
+		Model:     openai.GPT3Dot5Turbo,
+		Messages: []openai.ChatCompletionMessage{
+			{
+				Role: openai.ChatMessageRoleUser,
+				MultiContent: []openai.ChatMessagePart{
+					{
+						Type: openai.ChatMessagePartTypeText,
+						Text: "Hello!",
+					},
+					{
+						Type: openai.ChatMessagePartTypeImageURL,
+						ImageURL: &openai.ChatMessageImageURL{
+							URL:    "URL",
+							Detail: openai.ImageURLDetailLow,
+						},
+					},
+				},
+			},
+		},
+	})
+	checks.NoError(t, err, "CreateAzureChatCompletion error")
+}
+
+func TestMultipartChatMessageSerialization(t *testing.T) {
+	jsonText := `[{"role":"system","content":"system-message"},` +
+		`{"role":"user","content":[{"type":"text","text":"nice-text"},` +
+		`{"type":"image_url","image_url":{"url":"URL","detail":"high"}}]}]`
+
+	var msgs []openai.ChatCompletionMessage
+	err := json.Unmarshal([]byte(jsonText), &msgs)
+	if err != nil {
+		t.Fatalf("Expected no error: %s", err)
+	}
+	if len(msgs) != 2 {
+		t.Errorf("unexpected number of messages")
+	}
+	if msgs[0].Role != "system" || msgs[0].Content != "system-message" || msgs[0].MultiContent != nil {
+		t.Errorf("invalid user message: %v", msgs[0])
+	}
+	if msgs[1].Role != "user" || msgs[1].Content != "" || len(msgs[1].MultiContent) != 2 {
+		t.Errorf("invalid user message")
+	}
+	parts := msgs[1].MultiContent
+	if parts[0].Type != "text" || parts[0].Text != "nice-text" {
+		t.Errorf("invalid text part: %v", parts[0])
+	}
+	if parts[1].Type != "image_url" || parts[1].ImageURL.URL != "URL" || parts[1].ImageURL.Detail != "high" {
+		t.Errorf("invalid image_url part")
+	}
+
+	s, err := json.Marshal(msgs)
+	if err != nil {
+		t.Fatalf("Expected no error: %s", err)
+	}
+	res := strings.ReplaceAll(string(s), " ", "")
+	if res != jsonText {
+		t.Fatalf("invalid message: %s", string(s))
+	}
+
+	invalidMsg := []openai.ChatCompletionMessage{
+		{
+			Role:    "user",
+			Content: "some-text",
+			MultiContent: []openai.ChatMessagePart{
+				{
+					Type: "text",
+					Text: "nice-text",
+				},
+			},
+		},
+	}
+	_, err = json.Marshal(invalidMsg)
+	if !errors.Is(err, openai.ErrContentFieldsMisused) {
+		t.Fatalf("Expected error: %s", err)
+	}
+
+	err = json.Unmarshal([]byte(`["not-a-message"]`), &msgs)
+	if err == nil {
+		t.Fatalf("Expected error")
+	}
+
+	emptyMultiContentMsg := openai.ChatCompletionMessage{
+		Role:         "user",
+		MultiContent: []openai.ChatMessagePart{},
+	}
+	s, err = json.Marshal(emptyMultiContentMsg)
+	if err != nil {
+		t.Fatalf("Unexpected error")
+	}
+	res = strings.ReplaceAll(string(s), " ", "")
+	if res != `{"role":"user","content":""}` {
+		t.Fatalf("invalid message: %s", string(s))
+	}
+}
+
 // handleChatCompletionEndpoint Handles the ChatGPT completion endpoint by the test server.
 func handleChatCompletionEndpoint(w http.ResponseWriter, r *http.Request) {
 	var err error