Enable xla:gpu autocast for bfloat16 if not restricted (#5570)
* Enable autocast for XLA:GPU
* linter fix
* XLA autocast test for GPU and TPU
* linter fix
* Ensure that xla autocast is properly enabled for GPU and does not crash when torch cuda is not available.
* linter fix
* Add tests
* Support bf16
* linter fix
* exclude unsupported test cases
* increase GPU test timeout to 300
