feat(compute): add AllocDeviceFloat32 and CopyToDevice to FusedEncoderProvider

Enable callers to allocate persistent GPU buffers and upload weight data
for the fused encoder kernel. Without this, CPU-backed weight tensors
have no device pointer and the fused path always falls back to per-op.

- AllocDeviceFloat32: pool-managed GPU allocation
- CopyToDevice: host-to-device memcpy for float32 arrays

Author: dndungu

compute/fused_encoder.go

@@ -17,6 +17,13 @@ type FusedEncoderProvider interface {
 	// FusedEncoderAvailable returns true if the fused encoder kernel is loaded.
 	FusedEncoderAvailable() bool
 
+	// AllocDeviceFloat32 allocates numElements float32s on the GPU and returns
+	// the device pointer. Memory is pool-managed and freed when the engine closes.
+	AllocDeviceFloat32(numElements int) (unsafe.Pointer, error)
+
+	// CopyToDevice copies len(src) float32 values from host to a device pointer.
+	CopyToDevice(dst unsafe.Pointer, src []float32) error
+
 	// FusedEncoderForward executes one encoder layer forward pass.
 	// weights: [16]unsafe.Pointer to layer weights.
 	// bufs: [16]unsafe.Pointer to pre-allocated forward cache buffers.

compute/gpu_fused_encoder.go

@@ -5,6 +5,7 @@ import (
 	"unsafe"
 
 	"github.com/zerfoo/ztensor/internal/cublas"
+	"github.com/zerfoo/ztensor/internal/gpuapi"
 )
 
 // blasHandlePtr extracts the raw cuBLAS handle pointer from the BLAS interface.
@@ -68,5 +69,17 @@ func (e *GPUEngine[T]) FusedEncoderBackward(
 		totalRows, dModel, nHeads, headDim, ffnDim, bsC, numPatches, e.stream)
 }
 
+// AllocDeviceFloat32 allocates GPU memory for numElements float32 values.
+func (e *GPUEngine[T]) AllocDeviceFloat32(numElements int) (unsafe.Pointer, error) {
+	e.setDevice()
+	return e.pool.Alloc(e.deviceID, numElements*4)
+}
+
+// CopyToDevice copies float32 data from host to device.
+func (e *GPUEngine[T]) CopyToDevice(dst unsafe.Pointer, src []float32) error {
+	e.setDevice()
+	return e.runtime.Memcpy(dst, unsafe.Pointer(&src[0]), len(src)*4, gpuapi.MemcpyHostToDevice)
+}
+
 // Compile-time check that GPUEngine implements FusedEncoderProvider.
 var _ FusedEncoderProvider = (*GPUEngine[float32])(nil)