Allow only serializing external tensors

MLDrift's program cache serialization can fail if MLDrift changes
OR the GPU driver changes. It is not currently possible to detect
GPU driver changes on all devices so serializing the program
cache isn't always safe.

Allow serializing external tensors without serializing the program
cache.

PiperOrigin-RevId: 714088597

tflite/delegates/serialization.cc

@@ -238,14 +238,14 @@ TfLiteStatus SerializationEntry::GetData(TfLiteContext* context,
   }
 }
 
-SerializationEntry Serialization::GetEntryImpl(
-    const std::string& custom_key, TfLiteContext* context,
-    const TfLiteDelegateParams* delegate_params) {
+uint64_t Serialization::GetFingerprint(
+    const std::string& model_token, const std::string& custom_key,
+    TfLiteContext* context, const TfLiteDelegateParams* delegate_params) {
   // First incorporate model_token.
   // We use Fingerprint64 instead of std::hash, since the latter isn't
   // guaranteed to be stable across runs. See b/172237993.
   uint64_t fingerprint =
-      ::util::Fingerprint64(model_token_.c_str(), model_token_.size());
+      ::util::Fingerprint64(model_token.c_str(), model_token.size());
 
   // Incorporate custom_key.
   const uint64_t custom_str_fingerprint =
@@ -297,6 +297,14 @@ SerializationEntry Serialization::GetEntryImpl(
                                 partition_data.size() * sizeof(int32_t));
     fingerprint = CombineFingerprints(fingerprint, partition_fingerprint);
   }
+  return fingerprint;
+}
+
+SerializationEntry Serialization::GetEntryImpl(
+    const std::string& custom_key, TfLiteContext* context,
+    const TfLiteDelegateParams* delegate_params) {
+  uint64_t fingerprint =
+      GetFingerprint(model_token_, custom_key, context, delegate_params);
 
   // Get a fingerprint-specific lock that is passed to the SerializationKey, to
   // ensure noone else gets access to an equivalent SerializationKey.

tflite/delegates/serialization.h

@@ -193,6 +193,14 @@ class Serialization {
   Serialization(const Serialization&) = delete;
   Serialization& operator=(const Serialization&) = delete;
 
+  // Generate a unique fingerprint for the given model_token and custom_key.
+  // If context and delegate_params are provided, the fingerprint will be
+  // unique to the given context and delegate_params.
+  static uint64_t GetFingerprint(
+      const std::string& model_token, const std::string& custom_key,
+      TfLiteContext* context = nullptr,
+      const TfLiteDelegateParams* delegate_params = nullptr);
+
  protected:
   SerializationEntry GetEntryImpl(
       const std::string& custom_key, TfLiteContext* context = nullptr,