Revert "dist: coalesce grads into buckets for all-reduce (KI-5)"

This reverts commit b8b58212dc.

crates/xtrain-cuda/src/device.rs

@@ -14,15 +14,3 @@ pub fn set_device(device: u32) -> Result<()> {
 pub fn synchronize() -> Result<()> {
     error::check(unsafe { ffi::cudaDeviceSynchronize() })
 }
-
-/// Device-to-device copy of `count` bytes (`dst <- src`) on the same GPU. Issued
-/// on the null stream (like every other xtrain kernel), so it orders with the
-/// surrounding work. Used by the DDP bucketed all-reduce to pack/unpack grads
-/// into a flat scratch buffer.
-///
-/// # Safety
-/// `dst`/`src` must point to at least `count` valid bytes of device memory on the
-/// current device, with no overlap.
-pub unsafe fn copy_d2d(dst: *mut u8, src: *const u8, count: usize) -> Result<()> {
-    error::check(unsafe { ffi::cudaMemcpy(dst, src, count, ffi::CUDA_MEMCPY_D2D) })
-}