Skeleton for the avg-pool2d and upsample-nearest2d ops. (#337)

* Skeleton for the avg-pool2d and upsample-nearest2d ops.

* Preliminary conv2d support.

candle-core/src/op.rs

@@ -80,6 +80,21 @@ pub enum Op {
         stride: usize,
     },
 
+    #[allow(dead_code)]
+    Conv2D {
+        arg: Tensor,
+        kernel: Tensor,
+        padding: usize,
+        stride: usize,
+    },
+
+    AvgPool2D {
+        arg: Tensor,
+        kernel_size: (usize, usize),
+        stride: (usize, usize),
+    },
+    UpsampleNearest2D(Tensor),
+
     Cat(Vec<Tensor>, usize),
 
     #[allow(dead_code)] // add is currently unused.