[Quant][bc-breaking] Remove overwrite_output_observer (#88620)
Summary: When the BackendConfig was first introduced,
`overwrite_output_observer` and `overwrite_output_fake_quantize`
were added to ensure fixed qparams ops like `torch.nn.Sigmoid`
and `torch.nn.Tanh` used the correct observers and fake quantizes.
However, this is hacky because the BackendConfig should not set
the observer constructors themselves, but should instead specify
only requirements on the observers.
Later, https://github.com/pytorch/pytorch/pull/80184 added the
correct observers to `get_default_qconfig_mapping` along with
validation logic that throws an error if incorrect observers
were specified. With this change, we no longer need to overwrite
the observers from the BackendConfig, since we expect the user to
pass in the correct observers for these ops.
This commit removes these overwrite observer settings in the
BackendConfig. Instead, we represent the observer constraints for
fixed qparams ops through the existing DTypeWithConstraints
mechanism. Note that, however, to be consistent with other
DTypeWithConstraints checks, we no longer throw an error if an
incorrect observer is specified, but simply ignore the offending
QConfig and log a warning instead. This is the BC-breaking part
of the change.
BC-breaking notes:
```
from torch.ao.quantization.qconfig import default_qconfig
from torch.ao.quantization.quantize_fx import prepare_fx
model = ModelWithFixedQParamsOps()
qconfig_mapping = QConfigMapping().set_global(default_qconfig)
example_inputs = ...
prepare_fx(model, qconfig_mapping, example_inputs)
```
Before this commit, running the above leads to an exception
because the wrong observers are used for fixed qparams ops.
After this commit, the above will only encounter a warning,
and the fixed qparams ops will not be quantized. In both cases,
switching to `get_default_qconfig_mapping` will cause the
fixed qparams ops to be quantized.
Test Plan:
python test/test_quantization.py TestQuantizeFx
python test/test_quantization.py TestQuantizeFxOps
Reviewers: jerryzh168, vkuzo
Subscribers: jerryzh168, vkuzo
Pull Request resolved: https://github.com/pytorch/pytorch/pull/88620
Approved by: https://github.com/jerryzh168
