Add a simple implementation of cumsum. (#1334)

* Add a simple implementation of cumsum.

* Add another test.

candle-core/src/tensor.rs

@@ -2474,6 +2474,28 @@ impl Tensor {
         let t2 = t.reshape((n, 1))?.broadcast_as((n, n))?;
         t1.eq(&t2)?.to_dtype(dtype)
     }
+
+    /// Returns the cumulative sum of elements of the input tensor summed over the specified
+    /// dimension.
+    ///
+    /// This operation is most efficient when dim is the last dimension of the tensor.
+    pub fn cumsum<D: Dim>(&self, dim: D) -> Result<Self> {
+        let dim = dim.to_index(self.shape(), "cumsum")?;
+        let rank = self.rank();
+        if rank == 0 {
+            return Ok(self.clone());
+        }
+        let n_axis = self.dim(dim)?;
+        let triu = Tensor::triu2(n_axis, self.dtype(), self.device())?;
+        if rank == 1 {
+            self.unsqueeze(0)?.matmul(&triu)?.squeeze(0)
+        } else {
+            let last = rank - 1;
+            let t = self.transpose(dim, last)?;
+            let t = t.broadcast_matmul(&triu)?;
+            t.transpose(dim, last)
+        }
+    }
 }
 
 macro_rules! bin_trait {