NGTIndexer should be inherited from BaseNumpyIndexer
**Describe the bug**
In the [PR-599](https://github.com/jina-ai/jina/pull/599), we've refactored `NumpyIndexer` so that the user can switch between different Indexers during query. `NGTIndexer` needs to be adapted accordingly.

**Describe how you solve it**

```
class NGTIndexer(BaseNumpyIndexer):
```

---

<!-- Optional, but really help us locate the problem faster -->

**Environment**
<!-- Run `jina --version-full` and copy paste the output here -->

**Screenshots**
<!-- If applicable, add screenshots to help explain your problem. -->
diff --git a/jina/executors/indexers/vector/ngt.py b/jina/executors/indexers/vector/ngt.py
index ea4e1f1e7c7..d3347d7a251 100644
--- a/jina/executors/indexers/vector/ngt.py
+++ b/jina/executors/indexers/vector/ngt.py
@@ -4,10 +4,10 @@ __license__ = "Apache-2.0"
 from typing import Tuple
 
 import numpy as np
-from .numpy import NumpyIndexer
+from .numpy import BaseNumpyIndexer
 
 
-class NGTIndexer(NumpyIndexer):
+class NGTIndexer(BaseNumpyIndexer):
     """NGT powered vector indexer
 
     For more information about the NGT supported parameters and installation problems, please consult:
@@ -34,11 +34,8 @@ class NGTIndexer(NumpyIndexer):
         self.num_threads = num_threads
         self.epsilon = epsilon
 
-    def get_query_handler(self):
-        """Index all vectors , if already indexed return NGT Index handle """
-
+    def build_advanced_index(self, vecs: 'np.ndarray'):
         import ngtpy
-        vecs = super().get_query_handler()
         if vecs is not None:
             ngtpy.create(path=self.index_path, dimension=self.num_dim, distance_type=self.metric)
             _index = ngtpy.Index(self.index_path)
@@ -62,4 +59,4 @@ class NGTIndexer(NumpyIndexer):
             idx.append(index_k)
             dist.append(distance_k)
 
-        return self.int2ext_key[np.array(idx)], np.array(dist)
\\ No newline at end of file
+        return self.int2ext_key[np.array(idx)], np.array(dist)
['jina/executors/indexers/vector/ngt.py']