<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def import_extension_module(ext_name):
    '''
    Import an extension module by name.

    The extension modules are installed under the `nnabla_ext` package as
    namespace packages. All extension modules provide a unified set of APIs.

    Args:
        ext_name(str): Extension name. e.g. 'cpu', 'cuda', 'cudnn' etc.

    Returns: module
        An Python module of a particular NNabla extension.

    Example:

        .. code-block:: python

            ext = import_extension_module('cudnn')
            available_devices = ext.get_devices()
            print(available_devices)
            ext.device_synchronize(available_devices[0])
            ext.clear_memory_cache()

    '''
    import importlib
    try:
        return importlib.import_module('.' + ext_name, 'nnabla_ext')
    except ImportError as e:
        from nnabla import logger
        logger.error('Extension `{}` does not exist.'.format(ext_name))
        raise e