Load parameters from file.

        Parameters
        ----------
        filename : str
            Path to parameter file.
        ctx : Context or list of Context
            Context(s) initialize loaded parameters on.
        allow_missing : bool, default False
            Whether to silently skip loading parameters not represents in the file.
        ignore_extra : bool, default False
            Whether to silently ignore parameters from the file that are not
            present in this ParameterDict.
        restore_prefix : str, default ''
            prepend prefix to names of stored parameters before loading.

def load(self, filename, ctx=None, allow_missing=False,
             ignore_extra=False, restore_prefix=''):
        """Load parameters from file.

        Parameters
        ----------
        filename : str
            Path to parameter file.
        ctx : Context or list of Context
            Context(s) initialize loaded parameters on.
        allow_missing : bool, default False
            Whether to silently skip loading parameters not represents in the file.
        ignore_extra : bool, default False
            Whether to silently ignore parameters from the file that are not
            present in this ParameterDict.
        restore_prefix : str, default ''
            prepend prefix to names of stored parameters before loading.
        """
        if restore_prefix:
            for name in self.keys():
                assert name.startswith(restore_prefix), \
                    "restore_prefix is '%s' but Parameters name '%s' does not start " \
                    "with '%s'"%(restore_prefix, name, restore_prefix)
        lprefix = len(restore_prefix)
        loaded = [(k[4:] if k.startswith('arg:') or k.startswith('aux:') else k, v) \
                  for k, v in ndarray.load(filename).items()]
        arg_dict = {restore_prefix+k: v for k, v in loaded}
        if not allow_missing:
            for name in self.keys():
                assert name in arg_dict, \
                    "Parameter '%s' is missing in file '%s', which contains parameters: %s. " \
                    "Please make sure source and target networks have the same prefix."%(
                        name[lprefix:], filename, _brief_print_list(arg_dict.keys()))
        for name in arg_dict:
            if name not in self._params:
                assert ignore_extra, \
                    "Parameter '%s' loaded from file '%s' is not present in ParameterDict, " \
                    "choices are: %s. Set ignore_extra to True to ignore. " \
                    "Please make sure source and target networks have the same prefix."%(
                        name[lprefix:], filename, _brief_print_list(self._params.keys()))
                continue
            self[name]._load_init(arg_dict[name], ctx)