[XRT] Work on making XRT callable from Python via the TF eager API.

[XRT] Add an XRTReadToTensor operator that reads an XRT buffer into a host-memory Tensor. This is intended as a mirror image of XRTAllocateFromTensor, and is useful when interoperating between regular TF and XRT ops.
[XLA:Python] Add a helper for reading the ProgramShape of a computation, i.e., both argument and return value shapes. Minor refactoring to use Safe_PyObjectPtr.

PiperOrigin-RevId: 233734721