Update is_mlu_available

[ClowDragon]

to adapt torch_mlu main, the torch.is_mlu_available method is removed

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[ClowDragon]

@zhouzaida Can you help review this pr?

[HAOCHENYE]

@zhouzaida Can you help review this pr?

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Please sign the CLA first

[ClowDragon]

@HAOCHENYE I have signed the CLA

[yupeifengyannis]

change to

IS_MLU_AVAILABLE = torch.mlu.is_available()

[ClowDragon]

Done