caffe2/caffe2/operators/softmax_with_loss_op.cc: avoid shadowing warnings
caffe2/caffe2/operators/softmax_with_loss_op.cc: avoid shadowing warnings

Summary:
Fix warnings exposed by gcc-4.9.x's -Wshadow-compatible-local
(and/or the stricter -Wshadow-local) options.  Note that these
are both less onerous than -Wshadow.
I plan to enable one of them for all of fbcode, soon.

Rename inner "idx" to "k".

Differential Revision: D4216556

fbshipit-source-id: 5ee48751efd07838db24f56390730718ea031772

bwasti/caffe2,davinwang/caffe2,bwasti/caffe2,caffe2/caffe2,sf-wind/caffe2,xzturn/caffe2,sf-wind/caffe2,sf-wind/caffe2,Yangqing/caffe2,Yangqing/caffe2,Yangqing/caffe2,xzturn/caffe2,xzturn/caffe2,bwasti/caffe2,xzturn/caffe2,sf-wind/caffe2,pietern/caffe2,davinwang/caffe2,xzturn/caffe2,davinwang/caffe2,bwasti/caffe2,pietern/caffe2,sf-wind/caffe2,pietern/caffe2,Yangqing/caffe2,davinwang/caffe2,pietern/caffe2,pietern/caffe2,davinwang/caffe2,Yangqing/caffe2,bwasti/caffe2