Make `ComputationClient::Data` inherit from `torch::lazy::BackendData` (#5653)
* Start removing XLAData
* Make `ParseDeviceString` just parse
* formatting
* remove alias
* Remove `WrapXlaData` for a single data
* formatting
* Replace `UnwrapData` for single data
* Fix test build
* Remove duplicate virtual functions on Data
* Flatten overriding of Assign
* Move namespace for default device functions
* Move current device to XLA bridge
* Move default device calls
* formatting
* remove extra includes
* change function order slightly
* better errors
* Fix test build again (oops)
* Revert accidental changes
* static