[FIX] PadImpl_forward_kernels: 2D: fix issue about XY dimensions mismatch

Context

Issue: During Forward, pad node resize input with an X&Y dimensions mismatch. This error can be reproduced using X!=Y dimensions.

Solution: Modifying index calculation fixes the issue.

Related issue: #21 (closed)

Modified files

• PadImpl_forward_kernels.hpp , update index calculation with correct X&Y manipulation

@cmoineau @olivierbichler