rebase + nit (#2260)

Co-authored-by: Awni Hannun <awni@apple.com>

mlx/backend/cuda/copy/copy_contiguous.cu

@@ -0,0 +1,56 @@
+// Copyright © 2025 Apple Inc.
+
+#include "mlx/backend/cuda/copy/copy.cuh"
+
+#include <cooperative_groups.h>
+
+namespace mlx::core {
+
+namespace cu {
+
+namespace cg = cooperative_groups;
+
+template <typename In, typename Out, typename IdxT>
+__global__ void copy_s(const In* in, Out* out, IdxT size) {
+  IdxT index = cg::this_grid().thread_rank();
+  if (index < size) {
+    out[index] = CastOp<In, Out>{}(in[0]);
+  }
+}
+
+template <typename In, typename Out, typename IdxT>
+__global__ void copy_v(const In* in, Out* out, IdxT size) {
+  IdxT index = cg::this_grid().thread_rank();
+  if (index < size) {
+    out[index] = CastOp<In, Out>{}(in[index]);
+  }
+}
+
+} // namespace cu
+
+void copy_contiguous(
+    cu::CommandEncoder& encoder,
+    CopyType ctype,
+    const array& in,
+    array& out,
+    int64_t in_offset,
+    int64_t out_offset) {
+  encoder.launch_kernel([&](cudaStream_t stream) {
+    MLX_SWITCH_COPY_TYPES(in, out, InType, OutType, {
+      MLX_SWITCH_BOOL(out.data_size() > UINT32_MAX, LARGE, {
+        using IdxT = std::conditional_t<LARGE, int64_t, uint32_t>;
+        auto kernel = cu::copy_s<InType, OutType, IdxT>;
+        if (ctype == CopyType::Vector) {
+          kernel = cu::copy_v<InType, OutType, IdxT>;
+        }
+        auto [num_blocks, block_dims] = get_launch_args(kernel, out, LARGE);
+        kernel<<<num_blocks, block_dims, 0, stream>>>(
+            in.data<InType>() + in_offset,
+            out.data<OutType>() + out_offset,
+            out.data_size());
+      });
+    });
+  });
+}
+
+} // namespace mlx::core