CPU Context.

def context(type_config='float', **kw):
    """CPU Context."""
    backends = ['cpu:float']
    if type_config == 'half':
        backends = ['cpu:half', 'cpu:float']
    elif type_config == 'float':
        pass
    else:
        raise ValueError("Unknown data type config is given %s" % type_config)
    return nn.Context(backends, array_classes()[0], '')