[DDP] Rename num_iterations -> num_forward_calls (#103283)
This more accurately represents what we're counting. At iteration is a
forward + backward call, but here we're just counting forward calls. This makes
things less confusing in future diffs where we support DDP static graph
multiple forwards.
Differential Revision: [D46580601](https://our.internmc.facebook.com/intern/diff/D46580601/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/103283
Approved by: https://github.com/awgu
