NLPDDPPlugin find_unused_parameters is configurable
This PR makes `find_unused_parameters` user configurable in `NLPDDPPlugin` by setting a default dictionary with the current value (`False`) and updating that dictionary with user provided kwargs. This ensures that the value is always `False`, even if it changes from its current default (also `False`) in PyTorch's `DistributedDataParallel`.

An alternative option would be to remove it from the initialization call to `DistributedDataParallel` since it is already `False` by default in PyTorch. 

The reason for this request is that `find_unused_parameters` must be `True` for MegaMolBART.

I have read the contribution guidelines. All tests that are currently passing in `main` pass with this commit. (Note that two tests fail in NeMo `main`.) The code has been formatted with black and the commit is signed. This change was discussed with @ericharper several months ago.

Signed-off-by: Michelle Gill <mgill@nvidia.com>