Fix is_trained in IndexHNSWSQ (facebookresearch#3034)

Summary:
### Description
Even though `is_trained` is set as `true` in [IndexScalarQuantizer](https://github.com/facebookresearch/faiss/blob/main/faiss/IndexScalarQuantizer.cpp#L34). It is again overwritten as `false` in [IndexHNSW](https://github.com/facebookresearch/faiss/blob/main/faiss/IndexHNSW.cpp#L910) which is failing at this [validation check](https://github.com/facebookresearch/faiss/blob/main/faiss/IndexHNSW.cpp#L363) while ingesting vectors. Raising this PR with a small change to fix it.

Pull Request resolved: facebookresearch#3034

Reviewed By: pemazare

Differential Revision: D48900445

Pulled By: mdouze

fbshipit-source-id: 16b1cf17e9d8900c4a42956d466e30c76b13d064

faiss/IndexHNSW.cpp

@@ -899,7 +899,7 @@ IndexHNSWSQ::IndexHNSWSQ(
         int M,
         MetricType metric)
         : IndexHNSW(new IndexScalarQuantizer(d, qtype, metric), M) {
-    is_trained = false;
+    is_trained = this->storage->is_trained;
     own_fields = true;
 }