Implement copysign (#46396)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46396
Related #38349
[numpy](https://numpy.org/doc/stable/reference/generated/numpy.copysign.html?highlight=copysign#numpy.copysign)
- No in-place function
- No method
- Optional output
- Available: byte, char, bool, int, short, long, float, double, half
- Integral promoted to float
- Not available: float/double complex
`c = np.copysign(a, b)`
| a | b | c | a.grad |
| -1 | -1 | -1 | 1 |
| -0 | -1 | -0 | 0 |
| 0 | -1 | -0 | 0 |
| 1 | -1 | -1 | -1 |
| -1 | -0 | -1 | 1 |
| -0 | -0 | 0 | 0 |
| 0 | -0 | 0 | 0 |
| 1 | -0 | -1 | -1 |
| -1 | 0 | 1 | -1 |
| -0 | 0 | 0 | 0 |
| 0 | 0 | 0 | 0 |
| 1 | 0 | 1 | 1 |
| -1 | 1 | 1 | -1 |
| -0 | 1 | 0 | 0 |
| 0 | 1 | 0 | 0 |
| 1 | 1 | 1 | 1 |
This function becomes **non-differentiable** at `a=0` for any `b`. So, in my opinion, we may set the gradient for `a=0` to 0.
TODO:
- [x] test (cpu/gpu)
- [x] doc
- [x] ~kernel_vec~
Test Plan: Imported from OSS
Reviewed By: mruberry
Differential Revision: D24401366
Pulled By: ejguan
fbshipit-source-id: 3621c5ff74b185376a3705589983bb5197ab896d
