trunk/2bb2a9c522e7f944bf98c02c9b85fd959fa0bf0b: [MPS] Add heaviside (#195265)

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I have built this branch standalone and run its tests locally on Apple silicon before submitting. The description below was drafted with an AI assistant, so it is quoted rather than presented as my own prose. Why this is needed Requested on #141287 , where a user reports aten::heaviside.out missing and states that torch.heaviside on MPS-like devices would be useful for their astronomy work. Change Adds a native Meta…

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