Fix swiglu backwards return type (Dao-AILab#1337)

flash_attn/ops/activations.py

@@ -110,7 +110,7 @@ def sqrelu_bwd(g, x):
 }
 """
 swiglu_bwd_codestring = """
-template <typename T> T swiglu_bwd(T x, T y, T g, T& dx, T& dy) {
+template <typename T> void swiglu_bwd(T x, T y, T g, T& dx, T& dy) {
     float x_sigmoid = 1.0f / (1.0f + ::exp(-float(x)));
     dx = x_sigmoid * (1 + float(x) * (1.0f - x_sigmoid)) * float(g) * float(y);
     dy = float(x) * x_sigmoid * float(g);