logsumexp reduction support

[arogozhnikov]

It would be nice to have it, but there are problems with backends

• numpy.logaddexp.reduce is available (scipy.special.logsumexp is better, but I can't use it)
• tf.reduce_logsumexp is available
• problem cupy doesn'h have an implementation
• chainer has "chainer.functions.logsumexp", but we can't use if for cupy
• torch.logsumexp
• problem gluon and mxnet don't provide logsumexp
• keras provides keras.backend.logsumexp

Custom implementation through exp and max would take probably much more memory for a backward pass.