[quant] handle empty input in fused_moving_avg_obs_fake_quant op (#64829)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64829
If an empty input is passed in, the aminmax operator fails with a runtime error like
```
RuntimeError: aminmax(): cannot compute aminmax over an empty dimension as the operation has no identity.
```
To avoid this during training we just return the input if we find it to be empty
Test Plan:
python test/test_quantization.py TestFusedObsFakeQuant
Imported from OSS
Reviewed By: jingsh
Differential Revision: D30870879
fbshipit-source-id: 0cb4b187449a45a37150a77510d2292f93a7d1cd
