perf: streams / drop per-op sync

Default-stream kernels run in order and every host read goes through a
stream-ordered cudaMemcpy (to_device), so the per-op cudaDeviceSynchronize
after each kernel was pure overhead — remove all 21 in tensor.rs. Host
data is still correctly ordered by the D2H memcpy that reads it.

Also zero op-output buffers with cudaMemset (device-side, async) instead of
a blocking H2D memcpy of a host zero buffer on every allocation — that
copy was itself a hidden per-op sync point.

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

crates/xtrain-tensor/src/tensor.rs

@@ -153,7 +153,6 @@ impl Tensor {
                 std::ptr::null_mut(), // default stream
             );
         }
-        xtrain_cuda::device::synchronize().expect("scale kernel sync failed");
         out
     }
 
@@ -201,7 +200,6 @@ impl Tensor {
             0.0,
             out.data_ptr() as *mut f32,
         );
-        xtrain_cuda::device::synchronize().expect("matmul kernel sync failed");
         out
     }
 
@@ -229,7 +227,6 @@ impl Tensor {
                 std::ptr::null_mut(),
             );
         }
-        xtrain_cuda::device::synchronize().expect("transpose kernel sync failed");
         out
     }
 
@@ -277,7 +274,6 @@ impl Tensor {
             0.0,
             db.data_ptr() as *mut f32,
         );
-        xtrain_cuda::device::synchronize().expect("matmul_backward sync failed");
         (da, db)
     }
 
@@ -301,7 +297,6 @@ impl Tensor {
                 std::ptr::null_mut(),
             );
         }
-        xtrain_cuda::device::synchronize().expect("add sync failed");
         out
     }
 
@@ -319,7 +314,6 @@ impl Tensor {
                 std::ptr::null_mut(),
             );
         }
-        xtrain_cuda::device::synchronize().expect("mul sync failed");
         out
     }
 
@@ -342,7 +336,6 @@ impl Tensor {
                 std::ptr::null_mut(),
             );
         }
-        xtrain_cuda::device::synchronize().expect("add_bias sync failed");
         out
     }
 
@@ -363,7 +356,6 @@ impl Tensor {
                 std::ptr::null_mut(),
             );
         }
-        xtrain_cuda::device::synchronize().expect("sum_rows sync failed");
         out
     }
 
@@ -390,7 +382,6 @@ impl Tensor {
                 std::ptr::null_mut(),
             );
         }
-        xtrain_cuda::device::synchronize().expect("rms_norm sync failed");
         (y, inv_rms)
     }
 
@@ -427,7 +418,6 @@ impl Tensor {
                 std::ptr::null_mut(),
             );
         }
-        xtrain_cuda::device::synchronize().expect("rms_norm_backward sync failed");
         (dx, dgamma)
     }
 
@@ -444,7 +434,6 @@ impl Tensor {
                 std::ptr::null_mut(),
             );
         }
-        xtrain_cuda::device::synchronize().expect("silu sync failed");
         out
     }
 
@@ -462,7 +451,6 @@ impl Tensor {
                 std::ptr::null_mut(),
             );
         }
-        xtrain_cuda::device::synchronize().expect("silu_backward sync failed");
         dx
     }
 
@@ -485,7 +473,6 @@ impl Tensor {
                 std::ptr::null_mut(),
             );
         }
-        xtrain_cuda::device::synchronize().expect("rope sync failed");
         out
     }
 
@@ -506,7 +493,6 @@ impl Tensor {
                 std::ptr::null_mut(),
             );
         }
-        xtrain_cuda::device::synchronize().expect("rope_backward sync failed");
         dx
     }
 
@@ -525,7 +511,6 @@ impl Tensor {
                 std::ptr::null_mut(),
             );
         }
-        xtrain_cuda::device::synchronize().expect("softmax sync failed");
         out
     }
 
@@ -545,7 +530,6 @@ impl Tensor {
                 std::ptr::null_mut(),
             );
         }
-        xtrain_cuda::device::synchronize().expect("softmax_backward sync failed");
         dx
     }
 
@@ -571,7 +555,6 @@ impl Tensor {
                 std::ptr::null_mut(),
             );
         }
-        xtrain_cuda::device::synchronize().expect("cross_entropy sync failed");
         (probs, loss)
     }
 
@@ -592,7 +575,6 @@ impl Tensor {
                 std::ptr::null_mut(),
             );
         }
-        xtrain_cuda::device::synchronize().expect("cross_entropy_backward sync failed");
         dx
     }
 
@@ -640,7 +622,6 @@ impl Tensor {
                 std::ptr::null_mut(),
             );
         }
-        xtrain_cuda::device::synchronize().expect("embedding sync failed");
         out
     }
 
@@ -660,7 +641,6 @@ impl Tensor {
                 std::ptr::null_mut(),
             );
         }
-        xtrain_cuda::device::synchronize().expect("embedding_backward sync failed");
         dtable
     }
 
@@ -684,7 +664,6 @@ impl Tensor {
                 std::ptr::null_mut(),
             );
         }
-        xtrain_cuda::device::synchronize().expect("transpose_3d01 sync failed");
         out
     }