Tensor.abs() gives incorrect results on Complex64 when using MPS about pytorch HOT 6 CLOSED

For any user needing a quick (probably unsafe) fix, the following worked for my use case:

def patch_abs(t):
    if (t.device == torch.device("mps:0")) and (t.dtype == torch.complex64):
        return torch.sqrt(torch.pow(torch.real(t), 2) + torch.pow(torch.imag(t), 2) + 1e-12)
    return torch.abs(t)

torch.Tensor.abs = patch_abs

with torch.device("cpu"):
    print(torch.ones((2,), dtype=torch.complex64).abs())
    
with torch.device("mps"):
    print(torch.ones((2,), dtype=torch.complex64).abs())
    print(torch.tensor([1.0 + 0.0j, 0.0 + 10.0j, 100.0 + 0.0j, 1000.0 + 0.0j]).abs())

Output:

tensor([1., 1.])
tensor([1., 1.], device='mps:0')
tensor([   1.,   10.,  100., 1000.], device='mps:0')

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Grabbing for myself, wish there was a proper doc for https://developer.apple.com/documentation/metalperformanceshadersgraph/mpsgraph/3564540-absolutewithtensor?language=objc

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Hi @malfet! Wanted to add some context as I did have a brief discussion with some of the MPSGraph people regarding this issue and that particular op. My current hypothesis on what happens:

• If MPSGraph abs op gets passed an op that is data type Complex64, it will return a tensor of the same type. However if I print the type of the output from the CPU abs() (and the MPS abs for that matter) using the above snippet provided in the error it seems to be in float32. So probably the complex parts get dropped due to this conversion not being handled explicitly/correctly in the MPS abs implementation and we end up just interpreting the complex tensor as a float32 tensor. This would make complex 1+0i look like a float2 [1,0] tensor.
• If that's the case, we should check for complex data type in the abs op and explicitly take the real part of the complex output of the graph op absoluteWithTensor. There should be an API

-(MPSGraphTensor *) realPartOfTensor:(MPSGraphTensor *) tensor name:(NSString * _Nullable) name
that should do the trick.

Unfortunately I'm drowning in other work this week so I'm more than happy to let anyone take a shot at this if they'd like.

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Hi @bdhirsh !
First time contributor here. Could I work on this bug?

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@gambiTarun sorry, missed your comment. How familiar are you with ObjC/MPS framework?

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Hi @malfet, no worries! I’m not familiar with ObjC/MPS, but I’m willing to learn. If you could point me in the right direction for resolving this bug, or suggest some resources, I'd appreciate it!

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