Modular backends (#138)

* Add some trait to formalize backends.

* Use the generic backend trait.

candle-core/src/backend.rs

@@ -0,0 +1,71 @@
+use crate::{CpuStorage, DType, Layout, Result, Shape};
+
+pub(crate) trait BackendStorage: Sized {
+    type Device: BackendDevice;
+
+    fn try_clone(&self, _: &Layout) -> Result<Self>;
+
+    fn dtype(&self) -> DType;
+
+    fn device(&self) -> &Self::Device;
+
+    fn to_cpu_storage(&self) -> Result<CpuStorage>;
+
+    fn affine(&self, _: &Layout, _: f64, _: f64) -> Result<Self>;
+
+    fn elu(&self, _: &Layout, _: f64) -> Result<Self>;
+
+    fn sum(&self, _: &Layout, _: &[usize]) -> Result<Self>;
+
+    fn divide_by_sum_over_dim(&mut self, _: &Shape, _: usize) -> Result<()>;
+
+    fn to_dtype(&self, _: &Layout, _: DType) -> Result<Self>;
+
+    fn unary_impl<B: crate::op::UnaryOp>(&self, _: &Layout) -> Result<Self>;
+
+    fn binary_impl<B: crate::op::BinaryOp>(&self, _: &Self, _: &Layout, _: &Layout)
+        -> Result<Self>;
+
+    fn where_cond(&self, _: &Layout, _: &Self, _: &Layout, _: &Self, _: &Layout) -> Result<Self>;
+
+    fn conv1d(
+        &self,
+        _l: &Layout,
+        _kernel: &Self,
+        _kernel_l: &Layout,
+        _params: &crate::conv::ParamsConv1D,
+    ) -> Result<Self>;
+
+    fn embedding(&self, _: &Layout, _: &Self, _: &Layout) -> Result<Self>;
+
+    fn matmul(
+        &self,
+        _: &Self,
+        _: (usize, usize, usize, usize),
+        _: &Layout,
+        _: &Layout,
+    ) -> Result<Self>;
+
+    fn copy_strided_src(&self, _: &mut Self, _: usize, _: &Layout) -> Result<()>;
+}
+
+pub(crate) trait BackendDevice: Sized + std::fmt::Debug + Clone {
+    type Storage: BackendStorage;
+
+    // TODO: Make the usize generic and part of a generic DeviceLocation.
+    fn new(_: usize) -> Result<Self>;
+
+    fn location(&self) -> crate::DeviceLocation;
+
+    fn same_device(&self, _: &Self) -> bool;
+
+    fn zeros_impl(&self, _shape: &Shape, _dtype: DType) -> Result<Self::Storage>;
+
+    fn ones_impl(&self, _shape: &Shape, _dtype: DType) -> Result<Self::Storage>;
+
+    fn storage_from_cpu_storage(&self, _: &CpuStorage) -> Result<Self::Storage>;
+
+    fn rand_uniform(&self, _: &Shape, _: DType, _: f64, _: f64) -> Result<Self::Storage>;
+
+    fn rand_normal(&self, _: &Shape, _: DType, _: f64, _: f64) -> Result<Self::Storage>;
+}