Fix problem with nn.SpatialFractionalMaxPooling:clearState()
Fix problem with nn.SpatialFractionalMaxPooling:clearState()

:initSampleBuffer_() was producing an error when called (from :updateOutput()) after calling :clearState(). The randomSamples field was an empty tensor in that case.

Now :clearState() sets indices and randomSamples to nil, so the module is in the same state as after construction.

jhjin/nn,nicholas-leonard/nn,joeyhng/nn,diz-vara/nn,elbamos/nn,PraveerSINGH/nn,colesbury/nn,xianjiec/nn,kmul00/nn,sagarwaghmare69/nn,jonathantompson/nn,caldweln/nn,eriche2016/nn,apaszke/nn,andreaskoepf/nn