Proper seeding for DDP

[ajaysaini725]

If the seed is not set in hparams, it is randomly selected in __init__. Each DDP process, when it starts up, gets a different random seed.

The seed from the rank 0 process is saved in checkpoints

When resuming from a checkpoint, the seed from the rank 0 process is restored across all DDP processes.
This leads to inconsistent behavior, since the non-rank-0 process now resume with a different seed than they first trained with.

To fix: add the seed to the RNG state, and sync across all DDP processes