Summary:
This PR adds optional type to ATen native, autograd, JIT schema and Python Arg parser, closes #9513. It allows us to use optional default values (including None) for function signature and implementations like clamp, etc., and also let us remove the python_default_init hack.

Follow up:

remove python_default_init completely.
Pull Request resolved: pytorch/pytorch#12582

Differential Revision: D10417423

Pulled By: wanchaol

fbshipit-source-id: 1c80f0727bb528188b47c595629e2996be269b89

Summary:
For #6593 and #9515

This completes the support for optional<ScalarType> in native, JIT and autograd.

Note: Mostly following the existing implementation for optional<Scalar> that was added in #12582.

This PR introduces a way to make functions accept an optional dtype and it will unblock #9515 by allowing the `dtype` param for type promotion interface:
```
func: name(inputs, *, ScalarType? dtype=None, Casting casting=same_kind)
```

An alternative approach could have been using `ScalarType::Undefined` for the same purpose but without optional, though it would have been a bit hacky.
```
func: name(inputs, *, ScalarType dtype=Undefined, Casting casting=same_kind)
```

Here's an example use of this in action: 971f69e

There are already a bunch of native functions that were getting optional `dtype` through function overloading. #15133 is the attempt to migrate all of those. I will send those changes separately after this since some functions (e.g. sum) need quite a bit of change in the codebase. See the commits over there.
Pull Request resolved: #15154

Differential Revision: D13457760

Pulled By: tugrulates

fbshipit-source-id: 706134f0bd578683edd416b96329b49a1ba8ab48