Pjrt device attributes (#4403)
* Add GetDeviceAttributes for pjrt
* revert mnist test
* fix lint issues
* Add DeviceAttirbutes data type for cleanness
* remove comment
* add unit test for device attributes
* fix spacing
* lint fixes
* fix lint
* lint fixes
* lint fixes
* Fix type issues and return flat_has_map
* use StringToPjRtDevice while getting device attributes
