Implement approximate GELU kernels (#828)

csrc/activation.cpp

@@ -4,9 +4,25 @@ void silu_and_mul(
   torch::Tensor& out,
   torch::Tensor& input);
 
+void gelu_new(
+  torch::Tensor& out,
+  torch::Tensor& input);
+
+void gelu_fast(
+  torch::Tensor& out,
+  torch::Tensor& input);
+
 PYBIND11_MODULE(TORCH_EXTENSION_NAME, m) {
   m.def(
     "silu_and_mul",
     &silu_and_mul,
     "Activation function used in SwiGLU.");
+  m.def(
+    "gelu_new",
+    &gelu_new,
+    "GELU implementation used in GPT-2.");
+  m.def(
+    "gelu_fast",
+    &gelu_fast,
+    "Approximate GELU implementation.");
 }