Remove or move permute after mean

[DrJessop]

Summary:
If we have a permute -> unary chain -> mean, based on the reduction dims of the mean, we can either fully remove the preceding permute or move the permute after the mean.

Case 1: Dims after permute are still in same order with respect to each other, we can fully get rid of the permute and just update the reduction dims of the mean.

Case 2: Not case 1. In this case, it's better to move the permute after the mean, since the permute will operate on less data.

Differential Revision: D102268214

[pytorch-bot]

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[meta-codesync]

@DrJessop has exported this pull request. If you are a Meta employee, you can view the originating Diff in D102268214.

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