Device agnostic testing (#25870)
* adds agnostic decorators and availability fns
* renaming decorators and fixing imports
* updating some representative example tests
bloom, opt, and reformer for now
* wip device agnostic functions
* lru cache to device checking functions
* adds `TRANSFORMERS_TEST_DEVICE_SPEC`
if present, imports the target file and updates device to function
mappings
* comments `TRANSFORMERS_TEST_DEVICE_SPEC` code
* extra checks on device name
* `make style; make quality`
* updates default functions for agnostic calls
* applies suggestions from review
* adds `is_torch_available` guard
* Add spec file to docs, rename function dispatch names to backend_*
* add backend import to docs example for spec file
* change instances of to
* Move register backend to before device check as per @statelesshz changes
* make style
* make opt test require fp16 to run
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Co-authored-by: arsalanu <arsalanu@graphcore.ai>
Co-authored-by: arsalanu <hzji210@gmail.com>
