Python 3.10 adds `@dataclass(slots=True)` to auto-build the `__slots__` for a dataclass. This is really useful but we can't use it until 3.10 becomes our minimum version.
Copied the code for that functionality from python into a new decorator and ported it to use 3.8 syntax (removed use of `match`).
Usage:
```
@dataclass_slots
@dataclass
class X:
pass
```
is the same as (in py3.10):
```
@dataclass(slots=True)
class X:
pass
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/131014
Approved by: https://github.com/oulgen, https://github.com/eellison
