Added python implementation for GeLU (#1154)

* average callback and optimizers

* fixed links

* added SWA

* removed update, changed alias, 4 spaces

* added _gelu_py, test

* Delete average_optimizers_callback.ipynb

tensorflow_addons/activations/gelu.py

@@ -14,6 +14,7 @@
 # ==============================================================================
 
 import tensorflow as tf
+import math
 
 from tensorflow_addons.utils import types
 from tensorflow_addons.utils.resource_loader import LazySO
@@ -49,3 +50,13 @@ def _gelu_grad(op, grad):
     return _activation_so.ops.addons_gelu_grad(
         grad, op.inputs[0], op.get_attr("approximate")
     )
+
+
+def _gelu_py(x: types.TensorLike, approximate: bool = True) -> tf.Tensor:
+    x = tf.convert_to_tensor(x)
+    if approximate:
+        pi = tf.cast(math.pi, x.dtype)
+        coeff = tf.cast(0.044715, x.dtype)
+        return 0.5 * x * (1.0 + tf.tanh(tf.sqrt(2.0 / pi) * (x + coeff * tf.pow(x, 3))))
+    else:
+        return 0.5 * x * (1.0 + tf.math.erf(x / tf.cast(tf.sqrt(2.0), x.dtype)))

tensorflow_addons/activations/gelu_test.py

@@ -18,6 +18,7 @@
 import numpy as np
 import tensorflow as tf
 from tensorflow_addons.activations import gelu
+from tensorflow_addons.activations.gelu import _gelu_py
 from tensorflow_addons.utils import test_utils
 
 
@@ -51,6 +52,25 @@ def test_theoretical_gradients(self, dtype):
                 )
                 self.assertAllCloseAccordingToType(theoretical, numerical, atol=1e-4)
 
+    @parameterized.named_parameters(("float32", np.float32), ("float64", np.float64))
+    def test_same_as_py_func(self, dtype):
+        np.random.seed(100)
+        for _ in range(20):
+            self.verify_funcs_are_equivalent(dtype)
+
+    def verify_funcs_are_equivalent(self, dtype):
+        x_np = np.random.uniform(-10, 10, size=(4, 4)).astype(dtype)
+        x = tf.convert_to_tensor(x_np)
+        for approximate in [True, False]:
+            with tf.GradientTape(persistent=True) as t:
+                t.watch(x)
+                y_native = gelu(x, approximate=approximate)
+                y_py = _gelu_py(x, approximate=approximate)
+            self.assertAllCloseAccordingToType(y_native, y_py, atol=1e-4)
+            grad_native = t.gradient(y_native, x)
+            grad_py = t.gradient(y_py, x)
+            self.assertAllCloseAccordingToType(grad_native, grad_py, atol=1e-4)
+
 
 if __name__ == "__main__":
     tf.test.main()