[zero-3] add bwd support for list/dict types returned in fwd

[jeffra]

Currently if a forward pass returns a list or dict zero-3 will skip the backward pass on any Tensors that are embedded in the structures.

• add support for list return types
• add a unit test for common iterable data types (e.g., tuple, list, dict)
• add support for dict return types
• add support for objects that inherit from dict/list/tuple, e.g., https://github.com/huggingface/transformers/blob/main/src/transformers/modeling_outputs.py
• fix/test hf+ds regression

[samyam]

Looks good. Just left a few minor comments.