title: importError: cannot import name 'model_builder' from 'object_detection.builders' (unknown location)
body: <p>i got an error</p>
  <pre><code>Traceback (most recent call last):
    File &quot;c:/Users/User/Documents/Projects/Tensorflow/models/research/object_detection/builders/model_builder_tf2_test.py&quot;, line 24, in &lt;module&gt;
      from object_detection.builders import model_builder
  ImportError: cannot import name 'model_builder' from 'object_detection.builders' (unknown location) 
  </code></pre>
  <p>there is <strong>init</strong> in my folder
  i was running model_builder_tf2_test.py via cmd and this error happen</p>
  <p>the file script</p>
  <pre><code>import unittest
  
  import tensorflow.compat.v1 as tf
  
  from google.protobuf import text_format
  from object_detection.builders import model_builder
  from object_detection.builders import model_builder_test
  from object_detection.core import losses
  from object_detection.models import center_net_resnet_feature_extractor
  from object_detection.protos import center_net_pb2
  from object_detection.protos import model_pb2
  from object_detection.utils import tf_version
  </code></pre>