Fix scatter for empty indexes (#70662)
Summary:
This PR fixes an issue with `scatter` where the output is garbage for zero-sized indexes.
```py
import torch
null_index = torch.zeros((0, 4), dtype=torch.int64)
null_arr = torch.zeros((0, 4))
zeros_arr = torch.zeros((1, 4))
result = zeros_arr.scatter(0, null_index, null_arr)
print(null_index)
print(null_arr)
print(zeros_arr)
print(result)
```
```
tensor([], size=(0, 4), dtype=torch.int64)
tensor([], size=(0, 4))
tensor([[0., 0., 0., 0.]])
tensor([[1.7036e+19, 2.9965e+32, 3.9133e-14, 1.3585e-19]])
```
the out array is never filled if `index` arg has 0 elements.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70662
Reviewed By: dagitses
Differential Revision: D33476807
Pulled By: albanD
fbshipit-source-id: 97dbdd9c0133899e58828c43ecba81838807b8af
