[DDP] multiple forward support for static graph (#103487)
Adds support for multiple forward before bwd call for
static_graph=True.
There are 2 changes:
1) Change tracking of accounting of when to populate static grap related maps
from relying on forward iteration to backward calls
2) In DDP python, don't rely on num_forward iterations == 1 to enqueue the
delay allreduce. Instead use a flag.
Differential Revision: [D46673736](https://our.internmc.facebook.com/intern/diff/D46673736/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/103487
Approved by: https://github.com/awgu
