Unhide unique from C++, make unique partially scriptable (#15256)
Summary:
This PR does three things:
~~Allow `int64_t?` in function schema, which provide an elegant way of implementing null-able int arguments, as discussed in https://github.com/pytorch/pytorch/pull/15208#pullrequestreview-185230081~~
~~Originally implemented in https://github.com/pytorch/pytorch/pull/15235~~
~~Example:~~
```yaml
- func: myop(Tensor self, int64_t? dim=None) -> Tensor
variants: function
```
~~cc: zou3519~~
Edit: implemented in https://github.com/pytorch/pytorch/pull/15234
Previously tried in https://github.com/pytorch/pytorch/pull/12064. There was a problem that C++ does not have kwarg support, which makes it confusing to know whether `unique(t, 1)` actually means `unique(t, dim=1)` or `unique(t, sorted=1)`.
Now I think I have a better idea on how to implement this: there are two ATen operators: `unique` and `unique_dim`. `unique` has the same signature as in python, and exported to both python and C++. `unique_dim` has signature `unique_dim(tensor, dim, sorted=False, return_inverse=False)`, and only exported to C++, which could be used more naturally for a C++ user.
Differential Revision: D13540278
Pulled By: wanchaol
fbshipit-source-id: 3768c76a90b0881f565a1f890459ebccbdfe6ecd
