Enable bf16 option for XLA devices
# What does this PR do?

XLA devices like TPU and NeuronCore supports BF16 natively. This PR enables --bf16 option to work for XLA devices.

Since BF16 doesn't require gradient scaling, gradient scaling path is disabled for XLA devices.

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- [ ] Did you write any new necessary tests?


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@sgugger

_The documentation is not available anymore as the PR was closed or merged._<|||||>> I don't see how the gradient scaling path is disabled, could you share more information on that? A scaler is still defined at line 591.

Line 568 disables the code section that enables grad scaler when XLA device is detected (is_torch_tpu_available()).<|||||>This PR accidentally disabled gradient scaling when using FP16 on XLA devices. <|||||>Indeed. Do you want to make a PR with a fix @Lokiiiiii ?