[quant] handle empty input in fused_moving_avg_obs_fake_quant op (#64829)

[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