Add an erf based gelu op (#900)

* Erf based gelu.

* Add the erf backed gelu.

* Test the new gelu op (which is not gelu_new).

candle-core/src/op.rs

@@ -58,6 +58,7 @@ pub enum UnaryOp {
     Sqr,
     Sqrt,
     Gelu,
+    GeluErf,
     Relu,
     Tanh,
 }
@@ -325,6 +326,7 @@ pub(crate) struct Recip;
 pub(crate) struct Sqr;
 pub(crate) struct Sqrt;
 pub(crate) struct Gelu;
+pub(crate) struct GeluErf;
 pub(crate) struct Relu;
 pub(crate) struct Tanh;
 
@@ -621,6 +623,40 @@ impl UnaryOpT for Gelu {
     }
 }
 
+impl UnaryOpT for GeluErf {
+    const NAME: &'static str = "gelu_erf";
+    const KERNEL: &'static str = "ugelu_erf";
+    const V: Self = GeluErf;
+    #[inline(always)]
+    fn bf16(v: bf16) -> bf16 {
+        bf16::from_f64(Self::f64(v.to_f64()))
+    }
+    #[inline(always)]
+    fn f16(v: f16) -> f16 {
+        f16::from_f64(Self::f64(v.to_f64()))
+    }
+    #[inline(always)]
+    fn f32(v: f32) -> f32 {
+        Self::f64(v as f64) as f32
+    }
+    #[inline(always)]
+    fn f64(v: f64) -> f64 {
+        (crate::cpu::erf::erf(v / 2f64.sqrt()) + 1.) * 0.5 * v
+    }
+    #[inline(always)]
+    fn u8(_: u8) -> u8 {
+        0
+    }
+    #[inline(always)]
+    fn u32(_: u32) -> u32 {
+        0
+    }
+    #[inline(always)]
+    fn i64(_: i64) -> i64 {
+        0
+    }
+}
+
 impl UnaryOpT for Relu {
     const NAME: &'static str = "relu";
     const KERNEL: &'static str = "urelu";