'Arguments:
backend_module: NervanaGPU, NervanaCPU, NervanaMKL
dtype: np.float32, np.float16, etc
Returns:
Backend: the corresponding backend with certain default_dtype'

@staticmethod
def get_backend(backend_module, dtype):

    if (backend_module not in BackendPool.pools):
        BackendPool.pools[backend_module] = dict()
    pool = BackendPool.pools[backend_module]
    if (dtype not in pool):
        pool[dtype] = backend_module(default_dtype=dtype)
    be = pool[dtype]
    return be
