quant: ensure observers do not crash for empty Tensors (#49800)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49800
Ensures that having a Tensor with 0 elements does not crash observers.
Note: it's illegal to pass Tensors with 0 elements to reductions such
as min and max, so we gate this out before the logic hits min/max.
This should not be hit often in practice, but it's coming up
during debugging of some RCNN models with test inputs.
Test Plan:
```
python test/test_quantization.py TestObserver.test_zero_numel
```
Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D25693230
fbshipit-source-id: d737559697c98bd923356edacba895835060bb38
