Fix nn.DataParallel compatibility in PyTorch 1.5 (#4300)
* Test case for #3936
* multigpu tests pass on pytorch 1.4.0
* Fixup
* multigpu tests pass on pytorch 1.5.0
* Update src/transformers/modeling_utils.py
* Update src/transformers/modeling_utils.py
* rename multigpu to require_multigpu
* mode doc
