CUDA: fix logic for clearing padding with -ngl 0 (#13320)

ggml/src/ggml-cuda/mmq.cu

@@ -89,6 +89,16 @@ void ggml_cuda_mul_mat_q(
     const float * src1_d = (const float *) src1->data;
     float       *  dst_d = (float       *)  dst->data;
 
+    // If src0 is a temporary compute buffer, clear any potential padding.
+    if (ggml_backend_buffer_get_usage(src0->buffer) == GGML_BACKEND_BUFFER_USAGE_COMPUTE) {
+        GGML_ASSERT(ggml_is_contiguous(src0));
+        const size_t size_data  = ggml_nbytes(src0);
+        const size_t size_alloc = ggml_backend_buffer_get_alloc_size(src0->buffer, src0);
+        if (size_alloc > size_data) {
+            CUDA_CHECK(cudaMemsetAsync((char *) src0->data + size_data, 0, size_alloc - size_data, stream));
+        }
+    }
+
     const int64_t ne10_padded = GGML_PAD(ne10, MATRIX_ROW_PADDING);
 
     const int64_t s01 = src0->nb[1] / ts_src0;