fake_quantize: respect device affinity (#39031)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39031
Makes the eager mode QAT prepare logic respect device affinity.
This fixes the issue where a module is on `cuda:0`, and running
the QAT prepare script would add observers on `cpu`. Now it
will add them on the original device.
Test Plan:
```
python test/test_quantization.py TestDistributed.test_device_affinity
```
Imported from OSS
Differential Revision: D21729272
fbshipit-source-id: 5537bf3977ddc23412184941978bf0d1cc6fb479
