[quant][pt2e] Introduce QuantizationAnnotation API (#101708)
Summary:
This diff adds QuantizationAnnotation and also refactors the existing annotation to use this object
```
dataclass
class QuantizationAnnotation:
# How some input nodes should be quantized, expressed as QuantizationSpec
# a map from torch.fx.Node to QuantizationSpec
input_qspec_map: Dict[Node, QuantizationSpec]
# How the output of this node is quantized, expressed as QuantizationSPec
output_qspec: QuantizationSpec
class QuantizationSpec:
dtype: torch.dtype
is_dynamic: bool = False
quant_min: Optional[int] = None
quant_max: Optional[int] = None
qscheme: Optional[torch.qscheme] = None
ch_axis: Optional[int] = None
# TODO: follow up PR will add this
# Kind of observer such as MinMaxObserver, PerChannelHistogramObserver etc.
# observer_or_fake_quant_type: Union[ObserverBase, FakeQuantizeBase]
```
Example after full refactor:
```
int8_qspec = QuantizationSpec(dtype=torch.int8, ...)
weight_qspec = QuantizationSpec(dtype=torch.int8, ...)
conv_node["quantization_annotation"] = QuantizationAnnotation(
input_qspec_map={input_node: int8_qspec, weight_node: weight_qspec}
output_qspec=int8_qspec,
)
```
Note: right now input_qspec_map and output_qspec map are still using observer and fake quant constructors.
Follow up PR: change the input_qspec_map and output_qspec to use QuantizationSpec directly
Test Plan:
```
buck2 test mode/optcaffe2/test:quantization_pt2e -- --exact 'caffe2/test:quantization_pt2e - test_resnet18_with_quantizer_api (quantization.pt2e.test_quantize_pt2e.TestQuantizePT2EModels)'
```
Differential Revision: D45895027
Pull Request resolved: https://github.com/pytorch/pytorch/pull/101708
Approved by: https://github.com/andrewor14
