add acc_ops.max, acc_ops.maximum, consolidate acc_ops.min and acc_ops.minimum

add acc_ops.max, acc_ops.maximum, consolidate acc_ops.min and acc_ops.minimum

Summary:
This diff adds `acc_ops.max` and `acc_ops.maximum` support.
It further consolidates the logic for `acc_ops.min` and `acc_ops.minimum` to match the logic for max.

torch.max has three behaviors:
```1. max(input)
2. max(input, dim, keepdim=False, *, out=None)
3. max(input, other, *, out=None)
```

Likewise, `torch.min` has three identical behaviors.

I've chosen to implement each as an acc_op, then map to the appropriate one.

the third max function is effectively `torch.maximum`, so I've implemented it as that.

Reviewed By: yinghai, jfix71, 842974287

Differential Revision: D30551464

fbshipit-source-id: 0a2eec10e5185cbf7d9984eec3fd399b23528b2a