PR #11895: Offloading 3/3: enable rematerialization using xla flags
PR #11895: Offloading 3/3: enable rematerialization using xla flags

Imported from GitHub PR https://github.com/openxla/xla/pull/11895

This enables activation offloading automatically on GPU through HLO Rematerialization.
There is one xla flag needed to trigger rematerialization offloading:
1. A bool to enable host memory offloading. It's false by default to disable the offloading.

The other information is achieved by querying the GPU device:
2. Memory bandwidth in bytes per second.
3. The number of floating point operations on a device. 



Copybara import of the project:

--
46b6232dd0c9dd34d7b170f049e6074ccee002bd by Jane Liu <janeliu@nvidia.com>:

Offloading 3/3: enable rematerialization using a bool xla flag.
Query the memory bandwidth and flops from GPUs.

Merging this change closes #11895

FUTURE_COPYBARA_INTEGRATE_REVIEW=https://github.com/openxla/xla/pull/11895 from zhenying-liu:offloading-flags 46b6232dd0c9dd34d7b170f049e6074ccee002bd
