Merge C++ and Mesh implementation of most Mesh methods.

[copybara-service[bot]]

Merge C++ and Mesh implementation of most Mesh methods.

C++ becomes the source of truth for Mesh.

changes are in layout.py, tensor_layout.[cc|h], and the pywrap_dtensor_device.cc file.

Many attribute methods are converted to properties. If users are broken, will use forward fix.

I'll update Layout after this stablizes.