Fix portable pybindings COW ABI dependency

[JakeStevens]

Summary:
Avoid at::Tensor::data_ptr() in portable pybindings input conversion by using storage-level no-COW pointer access with the tensor storage offset. This prevents _portable_lib from depending on removed or renamed COW ABI symbols in newer Torch builds.

Test Plan:
cmake --build pip-out/temp.linux-x86_64-cpython-311/cmake-out --target portable_lib --config=Release -j8
nm -D pip-out/temp.linux-x86_64-cpython-311/cmake-out/_portable_lib.cpython-311-x86_64-linux-gnu.so | c++filt | rg "c10::impl::cow|materialize_cow|is_cow_data_ptr" -n || true
Direct-loaded rebuilt _portable_lib in Python and verified ExecuTorchModule is present.

[pytorch-bot]

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