keep_dims not respected when a reduction is over the entire input shape

[Bram Wasti]

Bug Description

As in title

flashlight/flashlight/fl/tensor/backend/af/ArrayFireReductions.cpp

Lines 184 to 192 in 5b44419

 if (isAllAxisReduction(input, axes)) { 
   // Reduce along all axes returning a singleton tensor 
   // TODO: modify this to af::mean<af::array> to take advantage of the 
   // ArrayFire reduce_all kernels once available 
   return toTensor<ArrayFireTensor>( 
       detail::condenseIndices( 
           af::mean(af::mean(af::mean(af::mean(toArray(input)))))), 
       /* numDims = */ 0); 
 } else { 

(current workaround in shumai: https://github.com/facebookresearch/shumai/blob/main/shumai/cpp/binding_gen.inl#L454-L458)