Fix weight cache bug (#18295)

Summary: XNNPACK sometimes calls weight cache look_up_or_insert with
ptr==nullptr. This is a bit weird - maybe a bug on the XNNPACK side?,
but it seems to do this when it's already hit the cache. Just return the
cached value for the key without validating contents in this case, since
we can't insert anything.

Differential Revision: D97006279

Author: GregoryComer

backends/xnnpack/runtime/XNNWeightsCache.cpp

@@ -206,9 +206,18 @@ size_t XNNWeightsCache::look_up_or_insert(
     size_t size) {
   size_t offset = context->look_up(context, cache_key);
 
+  // XNNPACK can call this with ptr==nullptr when it previously hit the cache
+  // and skipped packing. We can't validate against the ptr contents in this
+  // case, so just return the offset. This might actually be a bug in XNNPACK
+  // since calling look_up_or_insert with ptr==nullptr doesn't really make
+  // sense...
+  if (ptr == nullptr) {
+    return offset;
+  }
+
   if (offset != SIZE_MAX) {
     void* saved_ptr = context->offset_to_addr(context, offset);
-    if (0 == memcmp(ptr, saved_ptr, size)) {
+    if (saved_ptr != nullptr && 0 == memcmp(ptr, saved_ptr, size)) {
       return offset;
     }
     // Failure, cache is out of date