Fixing slice errors + comments.

2 files changed, 29 insertions, 3 deletions

diff --git a/candle-core/src/error.rs b/candle-core/src/error.rs
index f9e69122..30d06239 100644
--- a/candle-core/src/error.rs
+++ b/candle-core/src/error.rs
@@ -79,6 +79,13 @@ pub enum Error {
         nth_shape: Shape,
     },
 
+    #[error("Cannot divide tensor of shape {shape:?} equally along dim {dim} into {n_parts}")]
+    ShapeMismatchSplit {
+        shape: Shape,
+        dim: usize,
+        n_parts: usize,
+    },
+
     #[error("{op} can only be performed on a single dimension")]
     OnlySingleDimension { op: &'static str, dims: Vec<usize> },
 
diff --git a/candle-nn/src/var_builder.rs b/candle-nn/src/var_builder.rs
index 1466f6d0..3133f210 100644
--- a/candle-nn/src/var_builder.rs
+++ b/candle-nn/src/var_builder.rs
@@ -135,6 +135,17 @@ impl<'a> VarBuilder<'a> {
 }
 
 impl<'a> VarBuilder<'a> {
+    /// Get part of a tensor, typically used to do Tensor Parallelism sharding.
+    ///
+    /// If the tensor is of size (1024, 1024).
+    ///
+    /// `dim` corresponds to the dimension to slice into
+    /// `rank` is the rank of the current process
+    /// `world_size` is the total number of ranks in the process group
+    ///
+    /// `get_sharded("tensor", 0, 0, 2)` means `tensor.i((..512))`
+    /// `get_sharded("tensor", 0, 1, 2)` means `tensor.i((512..))`
+    /// `get_sharded("tensor", 1, 0, 2)` means `tensor.i((.., ..512))`
     pub fn get_sharded(
         &self,
         tensor_name: &str,
@@ -164,16 +175,24 @@ impl<'a> VarBuilder<'a> {
                 let dtype = view.dtype();
                 let mut shape = view.shape().to_vec();
                 let size = shape[dim];
+
+                if size % world_size != 0 {
+                    return Err(Error::ShapeMismatchSplit {
+                        shape: shape.into(),
+                        dim,
+                        n_parts: world_size,
+                    });
+                }
                 let block_size = size / world_size;
                 let start = rank * block_size;
                 let stop = (rank + 1) * block_size;
 
                 let iterator = if dim == 0 {
-                    view.slice(start..stop).unwrap()
+                    view.slice(start..stop).map_err(|_| Error::Msg(format!("Cannot slice tensor {tensor_name} ({shape:?} along dim {dim} with {start}..{stop}")))?
                 } else if dim == 1 {
-                    view.slice((.., start..stop)).unwrap()
+                    view.slice((.., start..stop)).map_err(|_| Error::Msg(format!("Cannot slice tensor {tensor_name} ({shape:?} along dim {dim} with {start}..{stop}")))?
                 } else {
-                    unimplemented!("Get sharded on dimensions != 0 or 1");
+                    candle::bail!("Get sharded on dimensions != 0 or 1")
                 };
 
                 shape[dim] = block_size;