with values set to None.
       For each entry whose value is set to None, we skip
       quantizing that entry in the model:

          qconfig_mapping = QConfigMapping
              .set_global(qconfig_from_prepare)
              .set_object_type(torch.nn.functional.add, None)  # skip quantizing torch.nn.functional.add
              .set_object_type(torch.nn.functional.linear, qconfig_from_prepare)
              .set_module_name("foo.bar", None)  # skip quantizing module "foo.bar"

    * *backend_config* (BackendConfig): A configuration for the
      backend which describes how
         operators should be quantized in the backend, this
         includes quantization mode support
         (static/dynamic/weight_only), dtype support (quint8/qint8
         etc.), observer placement for each operators and fused
         operators. See "BackendConfig" for more details

Returns:
      A quantized model (torch.nn.Module)
Return type: