Port token_model to TF2

This patch ports token_model to TF2. This patch is relatively simple as
most of the functionality provided by the token_model class is already
eager.

Pull Request: google#282

gematria/model/python/BUILD.bazel

@@ -185,9 +185,6 @@ gematria_py_test(
     size = "small",
     timeout = "moderate",
     srcs = ["token_model_test.py"],
-    tags = [
-        "manual",
-    ],
     deps = [
         ":oov_token_behavior",
         ":token_model",

gematria/model/python/token_model.py

@@ -136,6 +136,14 @@ def __init__(
       if self._oov_token is None:
         raise ValueError(f'Token {replacement_token} was not found in tokens.')
 
+    token_list_array = np.frombuffer(
+        b'\0'.join(token.encode('utf-8') for token in self._token_list),
+        dtype=np.uint8,
+    )
+    self._token_list_tensor = tf.constant(
+        token_list_array, name=TokenModel.TOKENS_TENSOR_NAME
+    )
+
     super().__init__(**kwargs)
 
   @property
@@ -195,16 +203,7 @@ def validate_basic_blockTokens(self, block: basic_block.BasicBlock) -> bool:
           return False
     return True
 
-  def _create_tf_graph(self):
+  def _forward(self, feed_dict: model_base.FeedDict):
     """See base class."""
-    super()._create_tf_graph()
-    # Convert the token list into an array of bytes. We need to go through NumPy
-    # because tf.constant() always treats a bytes() object as a string and can't
-    # use it with any other dtype.
-    token_list_array = np.frombuffer(
-        b'\0'.join(token.encode('utf-8') for token in self._token_list),
-        dtype=np.uint8,
-    )
-    self._token_list_tensor = tf.constant(
-        token_list_array, name=TokenModel.TOKENS_TENSOR_NAME
-    )
+    del feed_dict  # Unused.
+    raise NotImplementedError()

gematria/model/python/token_model_test.py

@@ -18,7 +18,7 @@
 from gematria.model.python import oov_token_behavior
 from gematria.model.python import token_model
 from gematria.testing.python import model_test
-import tensorflow.compat.v1 as tf
+import tensorflow as tf
 
 _OutOfVocabularyTokenBehavior = oov_token_behavior.OutOfVocabularyTokenBehavior
 
@@ -66,9 +66,7 @@ def test_token_list_tensor(self):
     model.initialize()
 
     self.assertSequenceEqual(model._token_list, self.tokens)
-    with self.session() as sess:
-      raw_token_list = sess.run(model.token_list_tensor)
-    token_list = raw_token_list.tobytes().split(b'\0')
+    token_list = bytes(model.token_list_tensor).split(b'\0')
     self.assertLen(token_list, len(set(token_list)))
     for token in self.tokens:
       self.assertIn(token.encode(), token_list)
@@ -167,5 +165,4 @@ def test_validate_basic_block_with_replacement_token(self):
 
 
 if __name__ == '__main__':
-  tf.disable_v2_behavior()
   tf.test.main()