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

This reverts commit b8b58212dc.

crates/xtrain-cuda/src/ffi.rs

@@ -5,7 +5,6 @@ pub type CudaStream = *mut c_void;
 
 pub const CUDA_MEMCPY_H2D: i32 = 1;
 pub const CUDA_MEMCPY_D2H: i32 = 2;
-pub const CUDA_MEMCPY_D2D: i32 = 3;
 
 pub const CUDA_SUCCESS: i32 = 0;
 pub const CUDA_ERROR_OUT_OF_MEMORY: i32 = 2;