Cache results of is_torch_tpu_available()
Author: comaniac
Pull request: yes

# What does this PR do?

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`xm.xla_device()` (called by `is_torch_tpu_available()`) hangs when calling multiple times but no XLA devices are available, and this results in Trainer hanging. Since currently `torch_xla` will be used as long as it is installed in the current active Python environment, I encountered this issue even when I only want to run the Trainer with PyTorch on GPU.

The detail reason behind `torch_xla` is still under investigation (see https://github.com/pytorch/xla/issues/3939).

To workaround this issue, this PR adds `lru_cache` to `is_torch_tpu_available()`, so that `xm.xla_device()` is guaranteed to be called only once when no XLA device is available.

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Comments:
_The documentation is not available anymore as the PR was closed or merged._

Not sure the reason of CI failure. It seems not relevant to this PR.

@LysandreJik thanks for the review and sure we could wait for @sgugger.
Meanwhile, do I need to do anything to fix the CI failure?

Thanks for bearing with us. The test failures are spurious and unrelated, so we can merge this.