Add Erf-based Gelu fusion rule (#2495)

Add Erf-based Gelu fusion rule

---------

Signed-off-by: Ganesan Ramalingam <grama@microsoft.com>

Author: gramalingam

onnxscript/rewriter/ort_fusions/gelu.py

@@ -6,7 +6,8 @@
 
 from onnxscript.rewriter import _fusion_utils, pattern
 
-_sqrt_two_over_pi = math.sqrt(2.0 / math.pi)
+_SQRT_TWO_OVER_PI = math.sqrt(2.0 / math.pi)
+_SQRT_TWO = math.sqrt(2.0)
 
 
 class GeluTanhFusion(pattern.RewriteRuleClassBase):
@@ -16,7 +17,7 @@ def pattern(self, op, x):
         t2 = op.Mul(0.044715, t1)
         t3 = op.Add(x, t2)
 
-        t4 = op.Mul(_sqrt_two_over_pi, t3)
+        t4 = op.Mul(_SQRT_TWO_OVER_PI, t3)
         t5 = op.Tanh(t4)
         t6 = op.Add(t5, 1)
         t7 = op.Mul(0.5, t6)
@@ -27,9 +28,23 @@ def rewrite(self, op, x):
         return op.FastGelu(x, _domain="com.microsoft")
 
 
-_rule = GeluTanhFusion.rule()
+class GeluErfFusion(pattern.RewriteRuleClassBase):
+    def pattern(self, op, x):
+        # GELU(x) = 0.5 * x * (1 + erf(x / sqrt(2)))
+        t1 = op.Div(x, _SQRT_TWO)
+        t2 = op.Erf(t1)
+        t3 = op.Add(t2, 1.0)
+        t4 = op.Mul(x, t3)
+        result = op.Mul(t4, 0.5)
+        return result
+
+    def rewrite(self, op, x):
+        return op.Gelu(x, _domain="com.microsoft")
+
 
-gelu_rules = pattern.RewriteRuleSet([_rule])
+_tanh_rule = GeluTanhFusion.rule()
+_erf_rule = GeluErfFusion.rule()
 
+gelu_rules = pattern.RewriteRuleSet([_tanh_rule, _erf_rule])
 
 fuse_gelu = _fusion_utils.apply_fusion_rules(gelu_rules)

onnxscript/rewriter/ort_fusions/gelu_test.py

@@ -52,6 +52,39 @@ def gelu_model(x):
         optimized_output = test_utils.ort_run("Optimized", model, input)
         test_utils.assert_allclose(original_output, optimized_output)
 
+    def test_gelu_erf_fusion(self):
+        _sqrt_two = math.sqrt(2.0)
+
+        @script()
+        def gelu_erf_model(x):
+            # GELU(x) = 0.5 * x * (1 + erf(x / sqrt(2)))
+            t1 = op.Div(x, _sqrt_two)
+            t2 = op.Erf(t1)
+            t3 = op.Add(t2, 1.0)
+            t4 = op.Mul(x, t3)
+            result = op.Mul(t4, 0.5)
+            return result
+
+        model_proto = gelu_erf_model.to_model_proto(
+            input_types=[FLOAT[10]], output_types=[FLOAT[10]]
+        )
+        model = ir.serde.deserialize_model(model_proto)
+
+        # Eliminate redundant CastLike ops:
+        optimize(model)
+
+        input = {"x": np.random.randn(10).astype(np.float32)}
+        original_output = test_utils.ort_run("Original", model, input)
+
+        fuse_gelu(model)
+        remove_unused_nodes(model)
+
+        self.assertEqual(len(model.graph), 1)
+        self.assertEqual(model.graph.node(0).op_type, "Gelu")
+
+        optimized_output = test_utils.ort_run("Optimized", model, input)
+        test_utils.assert_allclose(original_output, optimized_output)
+
 
 if __name__ == "__main__":
     unittest.main()