Add completion-with-tool example (sashabaranov#598)

As a user of this go SDK it was not immediately intuitive to me how to
correctly utilize the function calling capability of GPT4
(https://platform.openai.com/docs/guides/function-calling). While the
aformentioned link provides a helpful example written in python, I
initially tripped over how to correclty translate the specification of
function arguments when usingthis go SDK.

To make it easier for others in the future this commit adds a
completion-with-tool example showing how to correctly utilize the
function calling capability of GPT4 using this SDK end-to-end in a
CreateChatCompletion() sequence.

examples/completion-with-tool/main.go

@@ -0,0 +1,94 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"os"
+
+	"github.com/sashabaranov/go-openai"
+	"github.com/sashabaranov/go-openai/jsonschema"
+)
+
+func main() {
+	ctx := context.Background()
+	client := openai.NewClient(os.Getenv("OPENAI_API_KEY"))
+
+	// describe the function & its inputs
+	params := jsonschema.Definition{
+		Type: jsonschema.Object,
+		Properties: map[string]jsonschema.Definition{
+			"location": {
+				Type:        jsonschema.String,
+				Description: "The city and state, e.g. San Francisco, CA",
+			},
+			"unit": {
+				Type: jsonschema.String,
+				Enum: []string{"celsius", "fahrenheit"},
+			},
+		},
+		Required: []string{"location"},
+	}
+	f := openai.FunctionDefinition{
+		Name:        "get_current_weather",
+		Description: "Get the current weather in a given location",
+		Parameters:  params,
+	}
+	t := openai.Tool{
+		Type:     openai.ToolTypeFunction,
+		Function: f,
+	}
+
+	// simulate user asking a question that requires the function
+	dialogue := []openai.ChatCompletionMessage{
+		{Role: openai.ChatMessageRoleUser, Content: "What is the weather in Boston today?"},
+	}
+	fmt.Printf("Asking OpenAI '%v' and providing it a '%v()' function...\n",
+		dialogue[0].Content, f.Name)
+	resp, err := client.CreateChatCompletion(ctx,
+		openai.ChatCompletionRequest{
+			Model:    openai.GPT4TurboPreview,
+			Messages: dialogue,
+			Tools:    []openai.Tool{t},
+		},
+	)
+	if err != nil || len(resp.Choices) != 1 {
+		fmt.Printf("Completion error: err:%v len(choices):%v\n", err,
+			len(resp.Choices))
+		return
+	}
+	msg := resp.Choices[0].Message
+	if len(msg.ToolCalls) != 1 {
+		fmt.Printf("Completion error: len(toolcalls): %v\n", len(msg.ToolCalls))
+		return
+	}
+
+	// simulate calling the function & responding to OpenAI
+	dialogue = append(dialogue, msg)
+	fmt.Printf("OpenAI called us back wanting to invoke our function '%v' with params '%v'\n",
+		msg.ToolCalls[0].Function.Name, msg.ToolCalls[0].Function.Arguments)
+	dialogue = append(dialogue, openai.ChatCompletionMessage{
+		Role:       openai.ChatMessageRoleTool,
+		Content:    "Sunny and 80 degrees.",
+		Name:       msg.ToolCalls[0].Function.Name,
+		ToolCallID: msg.ToolCalls[0].ID,
+	})
+	fmt.Printf("Sending OpenAI our '%v()' function's response and requesting the reply to the original question...\n",
+		f.Name)
+	resp, err = client.CreateChatCompletion(ctx,
+		openai.ChatCompletionRequest{
+			Model:    openai.GPT4TurboPreview,
+			Messages: dialogue,
+			Tools:    []openai.Tool{t},
+		},
+	)
+	if err != nil || len(resp.Choices) != 1 {
+		fmt.Printf("2nd completion error: err:%v len(choices):%v\n", err,
+			len(resp.Choices))
+		return
+	}
+
+	// display OpenAI's response to the original question utilizing our function
+	msg = resp.Choices[0].Message
+	fmt.Printf("OpenAI answered the original request with: %v\n",
+		msg.Content)
+}