Traceback (most recent call last):
  File "../d.py", line 23, in <module>
    print(net(a, b))
  File "/home/bgawrych/Desktop/mxnet/python/mxnet/gluon/block.py", line 1407, in __call__
    return super().__call__(x, *args)
  File "/home/bgawrych/Desktop/mxnet/python/mxnet/gluon/block.py", line 716, in __call__
    _check_all_np_ndarrays(out)
  File "/home/bgawrych/Desktop/mxnet/python/mxnet/gluon/utils.py", line 480, in _check_all_np_ndarrays
    raise TypeError("Block's output ndarrays/symbols must be of type `mxnet.numpy.ndarray`"
TypeError: Block's output ndarrays/symbols must be of type `mxnet.numpy.ndarray` or `mxnet.symbol.numpy._Symbol`, while got output type <class 'mxnet.ndarray.ndarray.NDArray'>

import mxnet as mx
from mxnet.gluon import HybridBlock

mx.npx.set_np()

class TestBlock(HybridBlock):
    def __init__(self):
        super(TestBlock, self).__init__()
        self.d = mx.gluon.nn.Dense(1)
    def hybrid_forward(self, F, a, b, *args):
        res = self.d.hybrid_forward(F, a, b)
        return res

a = mx.np.random.uniform(low=-1, high=1, size=(1,1))
b = mx.np.random.uniform(low=-1, high=1, size=(1,1))

net = TestBlock()
net.initialize()
net.hybridize()

print(net(a, b))
net.optimize_for(a, b, backend="MKLDNN")
#print(net(a, b)) # <---- this line doesn't work now - we need to reload symbol with JSON
inputs, sym = net._cached_graph
sym = mx.sym.np._symbol.load_json(sym.tojson())
x = mx.gluon.SymbolBlock(sym, [mx.sym.var('data0'), mx.sym.var('data1')], net.collect_params())

print(x(a, b))