[Bugfix] fix rotary embedding test for _get_padded_tensor_shape

[luccafong]

Fix test failure below for test_rotary_embdding on padded tensor shape:

pytest -v tests/ke rnels/core/test_pos_encoding.py -k test_rotary_embedding

    def __torch_function__(self, func, types, args=(), kwargs=None):
        kwargs = kwargs or {}
        if func in _device_constructors() and kwargs.get('device') is None:
            kwargs['device'] = self.device
>       return func(*args, **kwargs)
E       RuntimeError: shape '[55, -1, 256]' is invalid for input of size 299200

========================= 1440 passed, 1921 deselected in 432.16s (0:07:12) =========================

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[LucasWilkinson]

Thanks for the PR! I think this might be cleaners; also ensures we test with padding: #18229