compat: fix pinned copy header usage

Replace the internal-only DenseTensor::memory_size() call in the compat mapped pinned helper with a header-visible byte count computation so downstream extensions that include compat headers keep compiling.

Co-authored-by: Codex <codex@openai.com>

Author: SigureMo

paddle/phi/api/include/compat/utils/mapped_pinned_tensor.h

@@ -95,7 +95,8 @@ inline paddle::Tensor _PD_CopyTensorToPinnedPlace(
     auto src_dense = std::dynamic_pointer_cast<phi::DenseTensor>(src.impl());
     if (src_dense && src_dense->meta().is_contiguous() &&
         src_dense->meta().offset == 0) {
-      auto bytes = src_dense->memory_size();
+      auto bytes = static_cast<size_t>(src_dense->numel()) *
+                   phi::SizeOf(src_dense->dtype());
       auto holder = _PD_CreateMappedPinnedAllocation(bytes, pinned_place);
       if (bytes > 0) {
         std::memcpy(holder->ptr(), src_dense->data(), bytes);