Allowed the gradInputs to be tables with tensors

@koraykv @DaniloRezende
Sometimes, it is needed to sum multiple gradInputs together.
Summing of tensors was supported.
It will be now supported to sum also tables of tensors.

The summing of gradInputs is needed, if an input is passed to multiple modules.
If the input to a module is a table of tensors, the gradInput is also a table of tensors.