Efficient implementation of `Tensor::ones()` for `metal` (#2512)

* WIP: hopefully better const impl

* with GPU

* More tests on

* Reverting primitive for

* Incorporating review changes - added check elem count check in kerner, using  for call strategy

* rustfmt ran

1 files changed, 32 insertions, 4 deletions

diff --git a/candle-core/src/metal_backend/mod.rs b/candle-core/src/metal_backend/mod.rs
index 69edd2d1..6f560c02 100644
--- a/candle-core/src/metal_backend/mod.rs
+++ b/candle-core/src/metal_backend/mod.rs
@@ -1917,10 +1917,38 @@ impl BackendDevice for MetalDevice {
         ))
     }
 
-    fn ones_impl(&self, shape: &Shape, dtype: DType) -> Result<Self::Storage> {
-        // TODO Is there a faster way ?
-        let cpu_storage = crate::cpu_backend::CpuDevice.ones_impl(shape, dtype)?;
-        self.storage_from_cpu_storage(&cpu_storage)
+    fn ones_impl(&self, shape: &Shape, dtype: DType) -> Result<MetalStorage> {
+        let name = match dtype {
+            DType::U8 => "fill_u8",
+            DType::U32 => "fill_u32",
+            DType::I64 => "fill_i64",
+            DType::F16 => "fill_f16",
+            DType::BF16 => "fill_bf16",
+            DType::F32 => "fill_f32",
+            DType::F64 => {
+                let cpu_storage = crate::cpu_backend::CpuDevice.ones_impl(shape, dtype)?;
+                return self.storage_from_cpu_storage(&cpu_storage);
+            }
+        };
+        let buffer = self.new_buffer(shape.elem_count(), dtype, "alloc-ones")?;
+        let command_buffer = self.command_buffer()?;
+        candle_metal_kernels::call_const_fill(
+            &self.device,
+            &command_buffer,
+            &self.kernels,
+            name,
+            shape.elem_count(),
+            &buffer,
+            1.,
+        )
+        .map_err(MetalError::from)?;
+
+        Ok(MetalStorage::new(
+            buffer,
+            self.clone(),
+            shape.elem_count(),
+            dtype,
+        ))
     }
 
     fn storage_from_slice<T: crate::WithDType>(&self, s: &[T]) -> Result<Self::Storage> {