Added xla friendly codepath to single_tensor_adamw (#102858)
There are extra graph compilations on XLA when beta{1,2} ** step get too small. This PR addresses this issue by making the `capturable` interface enabled for XLA, as well as switching to `torch.float_power` which preserves the same behaviour as the non-capturable flow on XLA.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/102858
Approved by: https://github.com/janeyx99, https://github.com/albanD
