Yanhua Sun b2f0928940 Add DeviceIndex xla op.
DeviceIndex op: given a list of device names, this operation returns the index of the device this op runs.  In the case of XLA, we are not executing on any device, we return the length of the list.

PiperOrigin-RevId: 317740778
Change-Id: I0679aa0adc5508b83502eee0d2044584577ed5b4
2020-06-22 15:06:38 -07:00
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2020-06-22 15:06:38 -07:00