Upsample grad (#1420)

* encode size of upsample in enum

* working convolution method for limited 2d kernels

* add test for sf 3 interpolation

* add higher dimensional tests, fix to work with multichannel input

* Remove commented out line.

---------

Co-authored-by: Laurent <laurent.mazare@gmail.com>

candle-core/src/op.rs

@@ -132,7 +132,11 @@ pub enum Op {
     },
 
     UpsampleNearest1D(Tensor),
-    UpsampleNearest2D(Tensor),
+    UpsampleNearest2D {
+        arg: Tensor,
+        target_h: usize,
+        target_w: usize,
+    },
 
     Cat(Vec<Tensor>, usize),