Polish the index-add op and use it in the index-select backprop (#218)

* Add the cpu version of index-add.

* More cpu support for index-add.

* Use index-add in the backprop.

candle-core/src/error.rs

@@ -112,11 +112,11 @@ pub enum Error {
         msg: &'static str,
     },
 
-    #[error("{op} invalid index {index} with src dim size {src_size}")]
+    #[error("{op} invalid index {index} with dim size {size}")]
     InvalidIndex {
         op: &'static str,
         index: usize,
-        src_size: usize,
+        size: usize,
     },
 
     #[error("cannot broadcast {src_shape:?} to {dst_shape:?}")]