example/numpy-ops/custom_softmax.py

# pylint: skip-file
import os
from data import mnist_iterator
import mxnet as mx
import numpy as np
import logging


class Softmax(mx.operator.CustomOp):
    def forward(self, is_train, req, in_data, out_data, aux):
        x = in_data[0].asnumpy()
        y = np.exp(x - x.max(axis=1).reshape((x.shape[0], 1)))
        y /= y.sum(axis=1).reshape((x.shape[0], 1))
        self.assign(out_data[0], req[0], mx.nd.array(y))

    def backward(self, req, out_grad, in_data, out_data, in_grad, aux):
        l = in_data[1].asnumpy().ravel().astype(np.int)
        y = out_data[0].asnumpy()
        y[np.arange(l.shape[0]), l] -= 1.0
        self.assign(in_grad[0], req[0], mx.nd.array(y))

@mx.operator.register("softmax")
class SoftmaxProp(mx.operator.CustomOpProp):
    def __init__(self):
        super(SoftmaxProp, self).__init__(need_top_grad=False)
    
    def list_arguments(self):
        return ['data', 'label']

    def list_outputs(self):
        return ['output']

    def infer_shape(self, in_shape):
        data_shape = in_shape[0]
        label_shape = (in_shape[0][0],)
        output_shape = in_shape[0]
        return [data_shape, label_shape], [output_shape], []

    def infer_type(self, in_type):
        return in_type, [in_type[0]], []

    def create_operator(self, ctx, shapes, dtypes):
        return Softmax()

# define mlp

data = mx.symbol.Variable('data')
fc1 = mx.symbol.FullyConnected(data = data, name='fc1', num_hidden=128)
act1 = mx.symbol.Activation(data = fc1, name='relu1', act_type="relu")
fc2 = mx.symbol.FullyConnected(data = act1, name = 'fc2', num_hidden = 64)
act2 = mx.symbol.Activation(data = fc2, name='relu2', act_type="relu")
fc3 = mx.symbol.FullyConnected(data = act2, name='fc3', num_hidden=10)
#mlp = mx.symbol.Softmax(data = fc3, name = 'softmax')
mlp = mx.symbol.Custom(data=fc3, name='softmax', op_type='softmax')

# data

train, val = mnist_iterator(batch_size=100, input_shape = (784,))

# train

logging.basicConfig(level=logging.DEBUG)

# MXNET_CPU_WORKER_NTHREADS must be greater than 1 for custom op to work on CPU
model = mx.model.FeedForward(
    ctx = mx.cpu(0), symbol = mlp, num_epoch = 20,
    learning_rate = 0.1, momentum = 0.9, wd = 0.00001)

model.fit(X=train, eval_data=val,
          batch_end_callback=mx.callback.Speedometer(100,100))