allow passing Python built-in types as dtypes (#21215)
Summary:
Another simple bit of syntax that NumPy supports and we don't.
Support int, float, and bool.
```python
>>> torch.randn((2,3), dtype=float)
tensor([[-0.1752, -0.3240, -0.6148],
[ 0.1861, 1.6472, 0.1687]], dtype=torch.float64)
```
A bit confusingly, Python's "float" actually means double, but nothing we can do about that.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21215
Differential Revision: D15697012
Pulled By: umanwizard
fbshipit-source-id: 9a38d960a610b8e67023486b0c9265edd3c22246
