Create a "PrepareCustomConfig" from a dictionary with the
      following items:
     "standalone_module_name": a list of (module_name,
     qconfig_mapping, example_inputs, child_prepare_custom_config,
     backend_config) tuples

     "standalone_module_class" a list of (module_class,
     qconfig_mapping, example_inputs, child_prepare_custom_config,
     backend_config) tuples

     "float_to_observed_custom_module_class": a nested dictionary
     mapping from quantization mode to an inner mapping from float
     module classes to observed module classes, e.g. {"static":
     {FloatCustomModule: ObservedCustomModule}}

     "non_traceable_module_name": a list of modules names that are
     not symbolically traceable "non_traceable_module_class": a
     list of module classes that are not symbolically traceable
     "input_quantized_idxs": a list of indexes of graph inputs
     that should be quantized "output_quantized_idxs": a list of
