quant: make various configs printable (#91419)
Summary:
Makes various quantization configs print out human readable values instead
of just the class name. This is useful when printing these configs out when
debugging.
Test plan:
test script
```
conf_1 = torch.ao.quantization.backend_config.backend_config.DTypeConfig()
print(conf_1)
conf_2 = torch.ao.quantization.backend_config.backend_config.BackendConfig()
print(conf_2)
conf_3 = torch.ao.quantization.backend_config.backend_config.BackendPatternConfig()
print(conf_3)
conf_4 = torch.ao.quantization.fx.custom_config.PrepareCustomConfig()\
.set_input_quantized_indexes([0])
print(conf_4)
conf_5 = torch.ao.quantization.fx.custom_config.ConvertCustomConfig()\
.set_preserved_attributes(['foo'])
print(conf_5)
conf_6 = torch.ao.quantization.fx.custom_config.FuseCustomConfig()\
.set_preserved_attributes(['foo'])
print(conf_6)
```
test script output
```
DTypeConfig(input_dtype_with_constraints=DTypeWithConstraints(dtype=None, quant_min_lower_bound=None, quant_max_
upper_bound=None, scale_min_lower_bound=None, scale_max_upper_bound=None, scale_exact_match=None, zero_point_exa
ct_match=None), output_dtype_with_constraints=DTypeWithConstraints(dtype=None, quant_min_lower_bound=None, quant
_max_upper_bound=None, scale_min_lower_bound=None, scale_max_upper_bound=None, scale_exact_match=None, zero_poin
t_exact_match=None), weight_dtype_with_constraints=DTypeWithConstraints(dtype=None, quant_min_lower_bound=None,
quant_max_upper_bound=None, scale_min_lower_bound=None, scale_max_upper_bound=None, scale_exact_match=None, zero
_point_exact_match=None), bias_dtype=None, is_dynamic=None)
BackendConfig({'name': '', '_pattern_complex_format_to_config': {}})
BackendPatternConfig({'observation_type': <ObservationType.OUTPUT_USE_DIFFERENT_OBSERVER_AS_INPUT: 0>})
PrepareCustomConfig({'input_quantized_indexes': [0]})
ConvertCustomConfig({'preserved_attributes': ['foo']})
FuseCustomConfig({'preserved_attributes': ['foo']})
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91419
Approved by: https://github.com/andrewor14