fix index for min

[jbencook]

Description

This PR fixes a bug in tf.min for the cpu backend where if you compute the min along an axis it returns the global min for each element, e.g.

tf.setBackend('cpu');
tf.tensor([[1, 2], [3, 4]]).min(0).print();

returns

Tensor
    [1, 1]

I'm not super familiar with these operations but the index of the flat typed array was fixed at 0 instead of using the offset that the other reduction operations use. Changing that seems to fix the behavior I was seeing.

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This change is 

[nsthorat]

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Review status: complete! 1 of 1 LGTMs obtained

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