[core ATen IR] Add decompositions for max, min, var_mean (#110906)
## Context
Add decompositions for `aten.max`, `aten.min`, and `aten.var_mean`. These operators follow a pattern of returning a tuple of outputs from two component operators:
```
aten.max(x) -> return aten.amax(x), aten.argmax(x)
aten.min(x) -> return aten.amin(x), aten.argmin(x)
aten.var_mean(x) -> return aten.var(x), aten.mean(x)
```
For `var_mean`, the `refs` implementation was doing something similar, so I changed it to call `torch.` ops instead like was done for other `refs` implementations previously. cc: @peterbell10 @lezcano
Note that Inductor lowers all these directly, so they are excluded from the Inductor decomp table.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/110906
Approved by: https://github.com/manuelcandales
