Work around compiler bug returning an optional unique_ptr.

It looks like some compilers (e.g. gcc-7) don't like returning
a moveable value directly when the return type is `std::optional`
(i.e. it fails to treat the returned value as an r-value and
automatically construct an optional instance around it).
Explicitly creating the `std::optional` and returning _that_
seems to work around the issue.

PiperOrigin-RevId: 507062621

tensorflow/core/kernels/fft_ops.cc

@@ -455,7 +455,8 @@ class FftPlanCache {
       cache_.erase(it);
     }
     --size_;
-    return value;
+    // Explicitly create an optional to avoid a compiler bug with gcc-7.
+    return std::optional<Value>(std::move(value));
   }
 
   // Inserts a plan into the cache as long as there is still capacity.