test: drop unused Var import in grad_accum

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/xtrain-train/tests/grad_accum.rs

@@ -20,7 +20,6 @@
 #![cfg(not(no_cuda))]
 
 use xtrain_autodiff::ops;
-use xtrain_autodiff::tape::Var;
 use xtrain_cuda::device;
 use xtrain_model::{Config, TinyTransformer, batched_ids_tensor};
 use xtrain_tensor::Device;