Refactoring the TF activations functions (#7150)

* Refactoring the activations functions into a common file

* Apply style

* remove unused import

* fix tests

* Fix tests.

tests/test_activations_tf.py

@@ -0,0 +1,24 @@
+import unittest
+
+from transformers import is_tf_available
+from transformers.testing_utils import require_tf
+
+
+if is_tf_available():
+    from transformers.activations_tf import get_tf_activation
+
+
+@require_tf
+class TestTFActivations(unittest.TestCase):
+    def test_get_activation(self):
+        get_tf_activation("swish")
+        get_tf_activation("gelu")
+        get_tf_activation("relu")
+        get_tf_activation("tanh")
+        get_tf_activation("gelu_new")
+        get_tf_activation("gelu_fast")
+        get_tf_activation("mish")
+        with self.assertRaises(KeyError):
+            get_tf_activation("bogus")
+        with self.assertRaises(KeyError):
+            get_tf_activation(None)