[PJRT] Allow GPU memory oversubscription when unified memory is enabled.
[PJRT] Allow GPU memory oversubscription when unified memory is enabled.

With this CL, we can enable GPU memory oversubscription via env flags.
For example, `TF_FORCE_UNIFIED_MEMORY=1 XLA_PYTHON_CLIENT_MEM_FRACTION=8.0` provides 8x the GPU memory to the program. The 'extra' memory is physically located on the other GPU devices and the host's RAM, with swapping done transparently by CUDA.

PiperOrigin-RevId: 383819164
Change-Id: Id139d3184d3a62983c1e86bf95ca4078a08db4f4

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