VLM: wire performance instrumentation and logits output (apple#72)

* VLM: wire performance instrumentation and logits output

- Report prompt throughput (prefill t/s) and generation throughput
  for VLM inference, matching the LLM path performance summary
- Wire --print-logits for VLM: shows top-5 token probabilities per step
- Wire --save-logits for VLM: saves top-K logits to JSON file
- Make TokenLogits and TopLogitEntry properties public (needed by runner)

Tested with LLaVA-1.5-7B bundle: prompt 590 tokens at 579 t/s,
generation 20 tokens at 19.4 t/s. Logits JSON output verified.

* VLM: wire performance instrumentation and logits output

- Report prompt throughput (prefill t/s) and generation throughput
  for VLM inference, matching the LLM path performance summary
- Wire --print-logits for VLM: shows top-5 token probabilities per step
- Wire --save-logits for VLM: saves top-K logits to JSON via LogitsWriter
- Make TokenLogits and TopLogitEntry properties public (cross-module access)

Tested with VLM bundle: prompt 590 tokens at 579 t/s,
generation 20 tokens at 19.4 t/s. Logits JSON output verified.

Author: stikves

swift/Sources/CoreAILanguageModels/Output/LogitsWriter.swift

@@ -77,16 +77,22 @@ public enum LogitsLength: Sendable {
 
 /// Represents logits information for a single generated token
 public struct TokenLogits: Sendable {
-    let tokenId: Int32
-    let tokenText: String
-    let topLogits: [TopLogitEntry]
+    public let tokenId: Int32
+    public let tokenText: String
+    public let topLogits: [TopLogitEntry]
+
+    public init(tokenId: Int32, tokenText: String, topLogits: [TopLogitEntry]) {
+        self.tokenId = tokenId
+        self.tokenText = tokenText
+        self.topLogits = topLogits
+    }
 }
 
 /// Represents a single entry in top-K logits
 public struct TopLogitEntry: Codable, Sendable {
-    let tokenId: Int32
-    let tokenText: String
-    let logit: Float
+    public let tokenId: Int32
+    public let tokenText: String
+    public let logit: Float
 
     enum CodingKeys: String, CodingKey {
         case tokenId = "token_id"

swift/Sources/Tools/llm-runner/LLMRunnerMain.swift

@@ -179,6 +179,9 @@ struct LLMRunner: AsyncParsableCommand, Sendable {
     @Option(name: .customLong("image"), help: "Path to an image file for vision-language models")
     var imagePath: String?
 
+    @Option(
+        help: "Maximum tiles for image splitting (overrides model config). 1 = single crop, no tiling.")
+
     @Flag(help: "Enable verbose logging")
     var verbose: Bool = false
 
@@ -886,27 +889,73 @@ struct LLMRunner: AsyncParsableCommand, Sendable {
         let inferenceID = InstrumentsProfiler.beginInference(
             promptTokens: vlmTokens.count, maxTokens: maxTokens)
 
+        await PerformanceMetrics.shared.setPromptTokenCount(vlmTokens.count)
+
         let tokenStream = try vlmEngine.generate(
             with: embeddedInput,
             tokens: vlmTokens,
             samplingConfiguration: samplingConfiguration,
-            inferenceOptions: InferenceOptions(maxTokens: maxTokens)
+            inferenceOptions: InferenceOptions(
+                maxTokens: maxTokens,
+                includeLogits: printLogits || saveLogits != nil
+            )
         )
 
+        CLILogger.log("VLM generate started, maxTokens=\(maxTokens)", component: "VLM")
+
+        // Prompt (prefill) timing — first token latency
+        var promptSpan: ProfileSpan? = InstrumentsProfiler.beginPrompt(tokens: vlmTokens.count, engine: "CoreAIVLM")
+        var extendSpan: ProfileSpan?
+        let needsLogits = printLogits || saveLogits != nil
+        let topKCount = saveLogitsLength.topKForFile ?? 5
+
         var generatedTokens: [Int] = []
+        var allTokenLogits: [TokenLogits] = []
         var previousText = ""
         for try await output in tokenStream {
+            if promptSpan != nil {
+                promptSpan?.end()
+                promptSpan = nil
+                extendSpan = InstrumentsProfiler.beginExtend(step: 0, tokens: 1)
+            }
+
             let token = output.tokenId
             if eosTokenIds.contains(token) { break }
             generatedTokens.append(Int(token))
+
+            if needsLogits, let logits = output.logits {
+                let floatLogits = logits.map { Float($0) }
+                let topEntries = LogitsWriter.extractTopK(
+                    from: floatLogits, tokenizer: tokenizer, k: topKCount)
+                let tokenText = tokenizer.decode(tokens: [Int(token)])
+                allTokenLogits.append(
+                    TokenLogits(
+                        tokenId: token, tokenText: tokenText, topLogits: topEntries))
+
+                if printLogits {
+                    let desc = topEntries.prefix(5).map {
+                        "[\($0.tokenId)]=\(String(format: "%.3f", $0.logit))"
+                    }.joined(separator: " ")
+                    print("\n  logits top5: \(desc)", terminator: "")
+                }
+            }
+
             let fullText = tokenizer.decode(tokens: generatedTokens)
             let delta = String(fullText.dropFirst(previousText.count))
             previousText = fullText
             print(delta, terminator: "")
             fflush(stdout)
         }
+        promptSpan?.end()
+        extendSpan?.end()
         print()
 
+        // Save logits to JSON if requested
+        if let path = saveLogits, !allTokenLogits.isEmpty {
+            try LogitsWriter.saveTopKJSON(tokenLogits: allTokenLogits, path: path)
+        }
+
+        // Record generation stats
         InstrumentsProfiler.endInference(
             generatedTokens: generatedTokens.count, signpostID: inferenceID)
         await PerformanceMetrics.shared.setGeneratedTokenCount(generatedTokens.count)